Prime Minister Malcolm Turnbull has warned Australians that the threat of terrorism is 'very real' after counter-terrorism agencies foiled a plot to 'bring down' a plane.

Four men were arrested on Saturday night when heavily armed counter-terrorism officers raided four Sydney properties over an alleged Islamic-inspired terrorist plot to blow up an aircraft.

A 'considerable amount of material' was seized from the property.

'The threat of terrorism is very real,' Prime Minister Turnbull said.

Prime Minister Malcolm Turnbull has warned Australians that the threat of terrorism is 'very real' after counter-terrorism agencies foiled a plot to 'bring down' a plane

Counter terrorism police (pictured) stormed four Sydney properties and arrested four men including one man (pictured) at Surry Hills after being alerted to an alleged plot to bring down an airplane with a 'bomb'

Police (pictured) found an 'explosive device' and part of Cleveland Street (pictured) in Surry Hills was cordoned off

Mr Turnbull said increased security measures have been placed on all Australian airports.

'Additional security measures were put in Sydney Airport on Thursday. These and further measures have been extended to airports at international and domestic airports around the country overnight,' Turnbull said.

People travelling on planes are recommended to allow extra time to get through security at airports for the following weeks with increased scrutiny nation-wide.

'Some of the measures will be obvious to the public, some will not be,' Turnbull said.

'Every day we are working tirelessly to keep you safe. Last night's disruptions are another very good example of the way our agencies are delivering our commitment to keep Australians safe from terror.'

The terrorist threat in Australia remains at 'probable' according to ASIO.

'Every day we are working tirelessly to keep you safe. Last night's disruptions are another very good example of the way our agencies are delivering our commitment to keep Australians safe from terror,' the Prime Minister said

The counter-terror raid in Surry Hills (pictured) took place just metres from the Redfern Mosque (pictured)

Forensic teams (pictured) and the bomb squad attended the scene during the Surry Hills (pictured) raid

Australian Federal Police Commissioner Andrew Colvin said officers became aware people in Sydney were planning to commit a terrorist attack using an 'improvised device'.

Commissioner Colvin said the four men in custody had not been charged by police.

'This is the start of a very long and protracted investigation,' he said.

Police believed the attack would target the Australian aviation industry at a major airport.

About 40 heavily-armed police stormed a Surry Hills home at 4pm before raiding properties in Lakemba, Punchbowl and Wiley Park in Sydney's west.

An Islamist cell is accused of planning to bring down a domestic flight with an 'explosive device' discovered at the Cleveland Street home.

AFP confirmed in a statement that four men were arrested as part of the investigation after they were alerted to the alleged terror plot.

Cleveland Street (pictured) is closed between Elizabeth St and Young St, and Goodlet Lane is also closed

Prime Minister Malcolm Turnbull released a statement on the raids, confirming the involvement of the Australian Federal Police, ASIO and NSW Police

An 'explosive device' was discovered at the Surry Hills property and deactivated by a bomb squad that was called to the scene.

One man draped in a bed sheet with a heavily-bandaged head was seen being led into an ambulance outside the Surry Hills property.

The bandaged man appeared to be distressed and bleeding from the head as he walked to a waiting ambulance.

He could be heard saying 'they bashed me.' When asked by who, he answered 'police.'