Two men and a teenager have pleaded guilty to plotting a terrorist attack on government buildings, including police headquarters, in Sydney almost three years ago.

Jibryl Almaouie and Sulayman Khalid on Monday in the NSW Supreme Court in Parramatta pleaded guilty to conspiring to carry out an act of terror in late 2014.

Another person under 18, who cannot be named, also pleaded guilty to the plan which involved carrying out attacks on government buildings with firearms.

Two men and a teenager have pleaded guilty to plotting a terrorist attack on government buildings, including police headquarters, in Sydney almost three years ago (pictured is the police raids in 2014)

Jibryl Almaouie and Sulayman Khalid (pictured on SBS) on Monday in the NSW Supreme Court in Parramatta pleaded guilty to conspiring to carry out an act of terror in late 2014

The trio that pleaded guilty on Monday did not stand when Justice Geoff Bellew entered the courtroom (pictured is Sulayman Khalid during a previous court appearance)

The two men and the teen who pleaded guilty on Monday are among six involved in the plot. The other three pleaded guilty earlier this year.

Three other men implicated in the plan, Mohammed Almarie, Farhad Said and Ibrahim Ghazzawi, all pleaded guilty in April to the lesser charge of knowingly making a document likely to facilitate a terrorist act.

The trio that pleaded guilty on Monday did not stand when Justice Geoff Bellew entered the courtroom. Neither did their families watching from the public gallery.

'Not guilty, oh sorry, I mean guilty,' Khalid said upon entering his plea to the charge.

Pictured are Jibryl Almaouie and his brother Mohamed Rashad Almaouie

Khalid and Almaouie were remanded in custody, while lawyers for the teen argued his strict bail should be continued, but Justice Bellew revoked it.

The teen smiled and blew a kiss to his family, who cried, as he was led away into custody.

The charge of conspiring to do acts in preparation of a terrorist act carries a maximum sentence of life in prison.

The three are due to reappear for sentencing submissions in early October.