A western Sydney teenager, who police claim tried to obtain a gun and bomb-making instructions, has pleaded guilty to planning for an Anzac Day terrorist attack.

The 16-year-old, who cannot be identified because of his age, was arrested on the eve of Anzac commemorations last year.

His intended target was said to have been a remembrance service in Sydney.

The maximum penalty for the offence is life in jail.

Police found handwritten notes at his home, declaring that sharia law should be established across the world.

Lawyer Upol Amin leaves Parramatta Children's Court today after entering the plea for his client. ( ABC News )

The boy's lawyer, Upol Amin, would not speculate on what would be an appropriate penalty.

"A plea has been entered and the matter is on it's way to finalisation," Mr Amin said.

The boy, who had left school and started an electrical apprenticeship, was at the time of his arrest enrolled in an intervention program designed to stop people being radicalised.

Today, wearing a maroon jumper and with his arms resting on a desk and his hands joined, the youth appeared on screen, forlorn but calm as his lawyer entered his plea on his behalf at Parramatta Children's Court.

The court heard the agreed facts for the case are not yet settled and the matter will return to the children's court in April.