A 22-year-old Sydney man has been charged with supplying the gun used by a teenager to kill police accountant Curtis Cheng in Parramatta almost two weeks ago.

Key points: Talal Alameddine, 22, charged with supply of a firearm

Talal Alameddine, 22, charged with supply of a firearm Raban Alou, 18, charged with terrorism-related offences

Raban Alou, 18, charged with terrorism-related offences The men will appear in court tomorrow

The men will appear in court tomorrow Both men refused bail

An 18-year-old man has also been charged with procuring a terrorist act in relation to the death.

Mr Cheng was shot dead by 15-year-old Farhad Jabar outside NSW police headquarters on October 2.

It was soon revealed the teenager had been radicalised, his attack politically motivated and linked to terrorism.

Talal Alameddine, 22, was arrested at his home in Merrylands this morning.

Raban Alou has been detained for a week. ( ABC: 7.30 )

He was arrested by "police attached to the Middle East Organised Crime Squad, along with investigators from the joint counter-terrorism teams," NSW Police Commissioner Andrew Scipione said.

Alameddine has been refused bail and will appear in Parramatta Local Court Friday.

Another man, Raban Alou, 18, has been charged with aid, abet, counsel and procure the commission of a terrorist act.

He has also been refused bail and will appear in Central Local Court on Friday.

Alou and Alameddine were both among four young men, aged between 16 and 22, who were arrested in raids that followed the fatal shooting.

Alameddine was released but Alou has been in custody for eight days since being arrested during a series of counter-terrorism raids last Wednesday.

The 15-year-old gunman, Farhad Jabar. ( Instagram )

The Australian Federal Police (AFP) successfully applied last week to keep Alou in custody for a further 100 hours.

On Monday, a Sydney court granted the AFP another 68 hours to interview the teenager.

In a statement, an AFP spokesperson confirmed an additional period of 68 hours of "specified time" was granted by a NSW District Court magistrate.

That court order expired on Thursday afternoon.

Commissioner Scipione said he was happy with the arrests.

"I'm encouraged by the fact that we've actually got people in custody," he said.

"I want to acknowledge and thank the Sydney joint counter-terrorism team for their dedication, professionalism and meticulous work around the clock."

Curtis Cheng (far left) was shot dead on October 2 at the Parramatta police headquarters. ( Supplied: NSW Police )

AFP Commissioner Andrew Colvin added "this is a very positive and welcome development in what is going to be a lengthy investigation".

"I have said repeatedly our first priority is the safety of the community and today the laying of charges against these two men should reassure the community, that where there is evidence of criminal offending, we will find it and act," Commissioner Colvin said.

"Today is the culmination of hundreds of hours of work by our officers whose professionalism and courage should be commended.

"There is much work still to be done and the ongoing security and safety of the Australian community depends on the vigilance of us all."