SYDNEY (AFP) - The Australian authorities on Wednesday (April 27) charged four men over a 2015 attack in which a teenage boy shot dead a police employee, alleging all were members of a terror organisation.

Farhad Jabar, 15, shot senior accountant Curtis Cheng, 58, outside the Sydney police headquarters where he worked last October. The teenager was killed in gunfire shortly afterwards.

New South Wales Deputy Commissioner of police Catherine Burn said the four had been charged with conspiring to carry out an act in preparation for, or planning, a terrorist act.

"We do allege that these four people acted together in concert and the fifth person, the 15-year-old, was a part of that," she told a press conference.

She said the charges reflect "the criminality of all of them involved to come together to actually prepare and plan to do an absolutely heinous criminal act which resulted in the murder of a police employee".

The men, aged 18, 20, 22 and 23, have all been charged with membership of a terrorist organisation, Ms Burn added, without revealing the name of the group or groups.

The 20-year-old is also accused of assisting the travel of the teenage gunman's sister to Syria on the day before the murder.

All four men are in custody and expected to appear in court on Thursday.

Australia has long been concerned about home-grown extremism and raised the terror threat alert level to high in September 2014.

Police have repeatedly warned about the young age of those apparently drawn to extremist ideology and incited to commit acts of terror.

On Sunday, the police charged a 16-year-old Sydney boy with preparing a terror attack linked to Anzac Day commemorations honouring servicemen and women. He has pleaded not guilty.