The 16-year-old, who cannot be named for legal reasons, was arrested and charged by police last year on the eve of the April 25 commemorations.
He was remanded in custody after the guilty plea in Parramatta Children's Court, with the case due to resume next month.
The offence of planning a terrorism act carries a maximum penalty of life imprisonment.
Police have alleged the boy tried to get a gun and instructions on how to make a bomb, the Australian Broadcasting Corporation reported.
They said his plan was to target an Anzac Day remembrance event in the city.
Police say they also found handwritten notes in the boy's home allegedly declaring that Islamic sharia law should be established globally, the ABC added.
Canberra has become increasingly worried about homegrown extremism and officials say they have prevented 11 terror attacks on home soil in the past two years.
But several have taken place, including the murder of a Sydney police employee in 2015 by a 15-year-old boy, who was then killed in an exchange of gunfire with officers.
Parliament last year passed new legislation to lower from 16 to 14 the age at which people can be subject to a control order - which aims to prevent a terror attack by limiting a person's movements, communication and activity.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)