Three of four males arrested on Wednesday morning over last week's fatal terrorist attack at Parramatta police headquarters were targeted in Australia's biggest counter-terrorism raids a year ago.

Police arrested four males aged between 16 and 22 in counter-terror raids across western Sydney.

A fifth person, a 24-year-old man from Merrylands, was also arrested but was not detained as part of the joint counter-terror raids operation.

He was arrested as a result of an outstanding warrant for identity fraud and other fraud matters.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 8 minutes 59 seconds 8 m More terror arrests and police asked how a teen killer slipped their net ( Sean Rubinsztein-Dunlop and Dylan Welch )

Police allege the four men arrested in the counter-terrorism operation are directly linked to the fatal shooting of police accountant Curtis Cheng outside the Parramatta police HQ last Friday.

Eighteen-year-old Raban Alou was arrested at Lane Street, Wentworthville, the same home where his older brother Kawa was arrested in sweeping raids in September last year and released without charge.

Police also arrested a 16-year-old boy, who cannot be named for legal reasons and who was in the same year at Arthur Phillip High School as Farhad Jabar, the 15-year-old who shot Mr Cheng.

At Marsfield, police arrested another target of the September 2014 raids, 22-year-old Mustafa Dirani, a former student of Arthur Phillip High.

The 2014 raids were sparked by a phone call in which Mohammad Ali Baryalei allegedly directed the killing of a random member of the public. ( ABC, file photo )

The 16-year-old boy and two 22-year-old men who were arrested have since been released, while the 18-year-old remained in custody overnight.

Police also returned to the Guildford home of Omarjan Azari, who is in jail on a charge of conspiracy to murder after being arrested in last year's raids.

The 2014 raids were sparked by a phone call to Azari in which Australia's most senior Islamic State lieutenant, Mohammad Ali Baryalei, allegedly directed him to kill a random member of the public.

Members of the group who were raided in 2014 and Baryalei, who has since been killed in Syria, were known to congregate at the Parramatta mosque where Farhad spent several hours before he carried out last Friday's attack.

The 15-year-old was shot and killed by police at the scene of the shooting.

15yo gunman 'did not act alone'

Police are investigating where the gun came from and whether one of the four men arrested on Wednesday morning supplied it to the gunman.

NSW Police Force Deputy Commissioner Catherine Burn it was unclear whether or not the four arrested inspired the terrorist attack on Parramatta Police HQ.

Police said the 15-year-old gunman, Farhad Jabar, was politically motivated and had links to terrorism. ( Instagram )

"We have certain suspicions and we are clearly going to have to sift through all of this over the next couple of days," she said.

She said the motivation of the 15-year-old was still unknown but police definitely had the suspicion he did not act alone.

"What we are investigating is a terrorism offence so what we would suggest and we suspect is that there was some influence — whether it was ideologically, religious or politically motivated — that determined and influenced the 15-year-old to go and commit this horrendous act of violence," she said.

But Ms Burn said the teen gunman had "not been a target of ours and is not somebody we would have assessed as a threat".

Acting Deputy Commissioner Neil Gaughan told reporters no terrorist organisation had claimed responsibility for the attack.

More than 200 officers took part in the raids. ( ABC News: Rachel Pupazzoni )

Meanwhile, New South Wales Premier Mike Baird said "there's no doubt" Sydney had a problem with Islamic radicalisation.

"Importantly though, this is a minority. It is something that will be dealt with," he said.

"It will be dealt with on the basis of the Muslim community [and] it'll be done more broadly with the whole community."

Three arrested plus gunman went to same school

Mr Baird said he was concerned about religious radicalisation in the state's schools, but insisted the problem was not widespread.

Two of the males arrested on Wednesday, along with a teenager charged with threatening police on Tuesday, and the teenage gunman from the Parramatta attack all went to Arthur Phillip High School.

Mr Baird said he had asked the Education Department to accelerate the work it was doing to combat radicalisation among school students, but he downplayed the extent of the problem.

"There have been some isolated incidents. They are being dealt with by police, they are being dealt with by the Department of Education together and that's what we need to continue to see," Mr Baird said.

"We need to understand that we are in a new world. The risks that are emerging are new.

"But certainly my strong assurance to the people of NSW is that our schools are safe."

On Tuesday morning a student was stopped by police on his way to Arthur Phillip High School and arrested. ( ABC News: David Spicer )

Earlier, Federal Justice Minister Michael Keenan defended programs designed to stop young people becoming influenced by radical extremism.

"We are working to try and divert people if we think they are falling under the spell of ISIL in the Middle East," Mr Keenan told the ABC's AM program.

Mr Keenan said Australia was a world leader in the global response, and was also making inroads at a local level.

In a message to parents today, Arthur Phillip High School principal Lynne Goodwin said NSW Police had advised the school that "there is no ongoing threat as a result of last Friday's tragic event".

"I would like to reassure the whole Arthur Phillip High School community that the school continues to be in close liaison with the Department of Education and the NSW Police to uphold our exemplary levels of student safety and student wellbeing," Ms Goodwin said.

"Our school counsellors are available for all students, if required, today or in the future."

Police search a house and car in Marsfield during early raids across western Sydney. ( ABC News: Mazoe Ford )

Police reach out to Turkish counterparts to find gunman's sister

Farhad's sister flew out of the country, bound for the Middle East, a day before the NSW Police headquarters shooting.

The woman is a key focus of inquiries into the crime, with police suspicious of the timing of her travel.

"We have passed the details of that young lady on to the Turkish national police who will actively try and find her for us," Mr Gaughan said.

"I might stress at this time there is no suggestion that she has been involved in any criminal activity here, but obviously investigators are very keen to have a chat with her about what she knows about her brother's action."

Hundreds of police involved in dawn raids

Map Map showing approximate location of properties targeted by police in raids

More than 200 officers took part in Wednesday morning's raids, launched at 6:00am AEDT.

Sorry, this video has expired Woman at Merrylands home raided by police lashes out at media

Neighbours living across the road from the Marsfield property confirmed it was not the first time police had visited the home.

Elaine Archer told the ABC when she saw police vehicles she thought to herself, "Ok, here we go again".

"We went through it the last time they raided the same house," Ms Archer said.

The operation followed Tuesday night's raid on the home of an Arthur Phillip High teenager who allegedly used his Facebook page to threaten police.