A lone gunman shot dead a civilian police IT expert at close range outside a force headquarters in a targeted attack today which has been described as a 'brutal' and 'callous murder'.

The assailant, dressed all in black, fired a number of shots at special constables guarding the NSW Police station in Parramatta, Sydney, before he was gunned down and killed by one of the officers.

Police Commissioner Andrew Scipione said the civilian employee was 'simply leaving work' when he was shot in the back of the head by the gunman who was wearing 'dark trousers and a flowing top'.

He refused to be drawn on whether the double shooting was terror-related, but admitted that officers from within the counter-terrorism command were working alongside the homicide squad.

The commissioner revealed a number of warnings had been sent round to police in the past two years reminding them to be 'vigilant' about attacks, but assured that the people of NSW were 'safe'.

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A civilian IT expert working for police is believed to have been shot dead after a lone gunman opened fire outside a force headquarters in Sydney, pictured officers gather around a white sheet covering a body

The gunman has also reportedly been killed after an exchange of gunfire with special constables who guard the entrance of the main station in Parramatta

Dozens of officers were seen searching the area where the double shooting took place today

The black-clad assailant fired a number of shots at special constables guarding the NSW Police station in Parramatta today before he was gunned down and killed by one of the officers

He said: 'I have viewed a number of pieces of footage, I can tell you that this was a brutal crime. It was a terrible crime.

'We're attempting to identify a man who was seen to approach the victim and discharge one single shot. Subsequently the assailant remained in the street here in Charles Street before he fired several further shots at a special constable.

'A number of special constables came out of the building and as they've emerged they've come under fire.

'In the exchange that followed the gunman was shot and killed. An employee of the NSW police force has been callously murdered here today. This is a very sobering time for us.'

The gunman, who has been described as of Middle Eastern appearance, reportedly walked past one female police officer before opening fire, according to the Daily Telegraph.

He allegedly acted 'aggressively' after shooting the IT worker at around 4.30pm and was seen screaming and brandishing his gun in the street.

The assailant was then killed in a shoot-out with three special constables just metres from the police station.

Commissioner Scipione said it was likely the gunman waited around after the murder in order to 'commit suicide by cop'.

It was previously reported that there had been increased 'chatter' in the past week about a possible attack on the Parramatta headquarters.

Sources said the building had been 'cased' and that every officer had been ordered to wear their guns on them at all times this week, even while at their desks.

Police Commissioner Andrew Scipione said the gunman, who was wearing dark trousers and a flowing top, shot the employee on his way home from work

Both bodies (circled in red) were still on the scene at 11pm, draped in white sheets and just metres apart

When asked whether police were warned about a possible attack at the station, Commissioner Scipione revealed there had been a number of alerts in 2014 and 2015, pictured officers on the scene

Detectives believe the civilian worker was 'deliberately targeted' and said he was shot at almost point-blank range

Dozens of people were seen waiting outside their homes tonight after several buildings near the scene were evacuated

And when questioned about whether police were aware on a possible attack at the station, Commissioner Scipione revealed a numthere had been a number of alerts in 2014 and 2015.

'There has been activity around a number of locations in NSW, they're the things we communicate to our ­officers,' Commissioner Scipione said.

'We have drawn officers back to the special warnings which are contained within alert 2015.

'We've refreshed that alert and yet again highlighted the importance of remaining vigilant and being ready to respond should they have to at any location but particularly around police stations. I want to ensure that we don't jump to conclusions, as I've said.

'I've indicated that but we're keeping an open mind. At this stage we've got nothing to link this event to any terrorist-related activity but we could not say that that wasn't the case. So clearly you would understand we have officers from within the counter-terrorism command.'

He was also quizzed about whether staff had allegedly been sent a number of emails warning about men who had been taking photographs of the building's entrance.

Commissioner Scipione said detectives believed the civilian worker was 'deliberately targeted' and revealed he had been shot at almost point-blank range.

'The footage that I've viewed shows that this particular man was certainly targeted,' he said.

But he did not elaborate on whether he was known to the gunman or whether he was attacked because of his IT activities.

Police currently believe the gunman was not working with anyone else, but have not ruled out the possibility there there may be others involved.

The commissioner praised the bravery of the officers who put themselves in the firing line and offered his condolences to the family of the public servant who lost his life.

Speaking to the media at around 9pm today, he said the relatives of the public servant had not yet been informed and would not go into any details about his identity.

Witnesses have reported seeing two bodies lying on the ground covered in white sheets (pictured)

An ambulance NSW spokeswoman said paramedics were on the scene at Charles St, in the city's CBD

Detectives have not yet established the identity of the deceased, according to a spokesman

Detectives have launched a 'critical incident investigation' and confirmed that two people had been killed after a number of shots were fired

'This man was simply leaving work this afternoon and he was gunned down. He was murdered on this street, this very street,' he said.

The attack occurred outside a daycare centre used by police force families and the children were locked inside for four hours after the shooting with a dead body at their doorstep.

Detectives have launched a level one critical incident, the highest order they can give, and confirmed that two people had been killed after a number of shots were fired.

Witnesses have described seeing the gunman, who was dressed all in black, running down the street brandishing a pistol at 4.30pm today just before he was shot.

They also reported seeing two bodies lying on the ground covered in white sheets just metres apart.

Real estate agent Edwin Almeida said he saw a man with a gun screaming and pacing up and down outside the building on Charles Street.

He said he then saw the man lying on the ground with a police officer pointing a gun at him.

'We looked out the window, saw security guards and what appeared to be a plain clothes police officer with gun drawn pointing at the person that was now lying on the floor surrounded by a pool of blood,' he said.

He wrote on his Facebook page: 'Four five shots fired by man outside our office and in front of NSW police head quarters. Man shot down by guards and detectives.'

A man called Nathan told 2GB Radio that he saw a man lying on the street surrounded by blood.

'I saw the guy dressed in black on the pavement with blood everywhere,' he said.

Shopkeeper Sammy Shak told The Daily Telegraph he saw two bodies on the ground after hearing 'six shots at least'.

'WE'RE KEEPING AN OPEN MIND': COMMISSIONER'S COMMENTS ON MOTIVE Police Commissioner Andrew Scipione refused to be drawn on whether the double shooting was terror-related and said detectives do not yet know the motive. In his media briefing tonight, he said: 'We don't know the motive and we don't yet know who the gunman is but he has committed an appalling act of brutality. 'I want to ensure that we don't jump to conclusions, as I've said, we're keeping an open mind but there is an investigation that's on foot. 'At this stage we've got nothing to link this event to any terrorist-related activity but we could not say that that wasn't the case. 'So clearly you would understand we have officers from within the counter-terrorism command working alongside homicide but this is a homicide investigation led by homicide.' 'My message to the people of NSW is that they are safe. There is no threat that we're dealing with that at this stage we haven't resolved here. 'We will get to the bottom of this matter, the investigation will be conducted, it will be very thorough and we'll do that as soon as we possibly can.' Police Commissioner Andrew Scipione (pictured) refused to be drawn on whether the double shooting was terror-related and said detectives do not yet know the motive Advertisement

Witnesses have described seeing the gunman, who was dressed all in black, running down the street brandishing a pistol at 4.30pm today just before he was shot, pictured is the scene outside the station

Officers in body armour have been seen patrolling the Parramatta CBD and guarding train stations, pictured is Charles Street

A section of Charles Street (pictured) between Macquarie Street and Hassall Street has been cordoned off and is currently being manned by police

An investigation is believed to be underway into whether the shooter had been recently charged by a detective from one of the State Crime Command squads

'When I went out there was two bodies on the floor and there was cops everywhere all around the area and they told me to go inside the shop straight away,' he said.

Channel Seven helicopter pilot Andrew Millett said: 'We can see two bodies on the ground approximately 200m away from police headquarters'.

Ambulances were called to the scene at 4:35pm after 'reports of two patients', a spokeswoman said.

Finance worker Rizwan Shaikh, who lives opposite the police headquarters, said he heard the shooting.

'I finished work and was in the shower and I heard the gunshots,' Mr Shaikh told The Daily Telegraph.

'I heard six or seven gunshots and it was pretty loud. In two to three minutes there were cops everywhere.'

Miffy Hong, 33, said her mother called her just after 5pm to tell her she could see a body covered by a sheer near police headquarters.

'She told me come back I don't know what's happening, she doesn't speak English,' she said.

The NSW Police Force building is home to the State Crime Command, which includes the homicide, drug, Middle Eastern organised crime and gangs squads.

An investigation is believed to be underway into whether the shooter had been recently charged by a detective from one of the State Crime Command squads.

One witness (left) described hearing several shots and Dennis Entriken (right) raised fears about his three-year-old daughter who remains locked inside a childcare centre close to where the shooting took place

Several roads in Parramatta were blocked after the shooting and helicopters were seen circling overhead, pictured is Charles Street

Both bodies were still at the scene at around midnight, draped in white sheets and just metres apart.

They were just metres away from a childcare centre, a coffee shop and Parramatta Public School is located nearby.

Parents of young children, who were locked inside Goodstart Early Learning for four hours just metres from where the shooting took place, voiced fears about their welfare.

Dennis Entriken, whose three-year-old daughter was not allowed to leave for four hours, told Daily Mail Australia: 'It's very frustrating. One of the dead bodies is right out of the front of the chilcare centre.

'What did they see, what did they hear? Is she scared? Is she OK?

'They've told us she's safe which is good... it's the unknown which is the issue.

'If she saw nothing and she's blissfully unaware then that's good,' he said.

In his press conference tonight, Commissioner Scipione confirmed that all the children were safe.

He said: 'Everyone's safe, that's the good news. There was certainly no suggestion of anyone being injured there. That's certainly very pleasing to us.

'I've got to commend the brave actions of those officers. Those special Constables certainly put their lives on the line this afternoon. They acted in a way that was incredibly courageous and, you know, I'm just so pleased that they're all well but they were incredibly professional.'

Dozens of police officers were seen combing the area where the shooting took place from around 10pm, searching for clues.

Investigators also aimed huge spotlights at neighbouring apartments in their quest for clues.

Residents were evacuated from their homes in buildings nearby the police station and most were allowed to return just after 11pm.

Two people have been hurt and there are reports of a shooting outside police headquarters in Parramatta, in Sydney's west

Witnesses described hearing what sounded like a car tyre exploding before buildings near the headquarters were evacuated

Police set up a 2km exclusion zone after the shooting at around 4.30pm.

A section of Charles Street between Macquarie Street and Hassall Street has been been cordoned off and is currently being manned by police.

Officers in body armour have been seen patrolling the Parramatta CBD and guarding train stations.

Witnesses described hearing what sounded like a car tyre exploding before buildings near the headquarters were evacuated.

A critical incident team from the State Crime Command's Homicide Squad will conduct an investigation overseen by the Coroner. There will then be an inquest.

Commissioner Scipione said a major events team was looking at the incident with a view to the NRL grand final, but said security for the match at the ANZ Stadium on Sunday night was already high.

'My message to people across NSW is that there is nothing to fear. Go about your business including this weekend's major sporting events. We will do our job and get to the bottom of what has happened,' he said.