THE former head of the counterterrorism unit ­believes Australians fighting in Syria could become the equivalent of sleeper cells when they return home, waiting years to strike.

Peter Dein retired from the NSW Police Force last week after 41 years, ending his career as an assistant commissioner leading the fight against terrorism as boss of the NSW counterterrorism unit.

He echoed the concerns of other terror experts that the increasing number of Australians going to fight in areas such as Syria greatly increased the threat of an attack here.

media_camera Retired head of the NSW counterterrorism unit Peter Dein / Picture: John Fotiadis

But the qualified barrister warned that the threat of returning foreign fighters could remain for many years to come.

In 2005, nine Sydney men were convicted of ­plotting a terror attack after it was discovered that they were buying bomb-making components and believed to be ready to launch a terrorist attack.

“It is history repeating itself. Some of those convicted terrorists had fought overseas many years before.

“We are not talking about coming back and doing something bad in the next few months. They obtained skills in the late ’90s and planned their attacks in 2005.

media_camera Peter Dein during his time as a police officer / Picture: Supplied

“This (current threat) is not going to go away for years,” he said.

However, Mr Dein was loathe to use the term sleeper cells because “that is some one who has an intent to do something in the future. The returning foreign fighter may not have any plans but later something sparks him to act.”

During his six years working in counterterrorism, the retired assistant commissioner said it was often Sydney’s own Muslim community that helped keep the peace.

“When radical groups come out saying some of the most provocative things, it is the Muslim community itself that very often howls them down the loudest.’’

Mr Dein joined the force in 1973 when there was no such thing as a terrorism unit and started at Campsie police station before becoming a detective in 1979.

He worked on some of the city’s most high-profile cases, including the kidnapping and murder of Kerry Whelan in 1997.

“That is still one of the saddest cases of my career. I think kidnap and ransom is one of the cruellest crimes. especially when in this case the body of the loved one is not found.’’

The one major low point in his four decades with the police force was the royal commission into police corruption in 1996.

“We had to have it but it tarnished the reputation of detectives to the point where some young policemen didn’t want to join the detective ranks.

“Peter Ryan helped restore the name of the detectives. The structure was torn down and rebuilt into what we have now and I believe NSW is in very good hands,” he said.