Further charges have been laid against four Australians who plotted and conspired a terrorist attack against police accountant Curtis Cheng in Sydney last year.

Cheng, of Chinese-Australian descent, was shot and killed by a 15-year-old radicalized teenager Farhad Jabar outside the New South Wales state police's western Sydney headquarters in early October 2015. The boy was subsequently shot by officers guarding the building.

The four men aged between 18 and 23, already in custody on other terror-related offences, on Wednesday were charged in relation to conspiring in the preparation to commit a terror attack against Cheng.

Deputy Commissioner Catherine Burn told reporters in Sydney the men were regarded as the "principles", working with the 15-year-old, but would not rule out further charges against others.

The 18-year-old is accused of supplying the gun to the 15-year-old murderer, while a 20-year-old man has been charged in relation to organising Jabar's sister to travel to Syria the day before the attack.

The charges come just three days after another teenager was arrested and changed with plotting to commit a terror attack on the day Australians commemorate their armed forces.

"It is important that the community is assured that as of today there is no new specific threat," Burn said.

"However we will continue to investigate any threats that we come across."

Source: XINHUA