A 15-year-old boy in Sydney has been charged with planning a terror attack and has been denied bail by Judge James Viney.

The teenager is considered an 'unacceptable risk to the community' due to encrypted messages outlining threats against Jewish and Assyrian communities.

Five other teens are implicated alongside the accused, all facing charges of planning a terrorist act.

Despite arguments of exceptional circumstances, including the boy's behavioral issues and low self-esteem, the judge upheld the bail denial.

Evidence includes the teenager's discussions about using knives for an attack, hand-drawn ISIS flags found in his room, and a previous phone confiscation by his parents after a stabbing incident.

The defense intends to appeal the bail decision in the Supreme Court.

The boy is scheduled for a court return on 21 June as the case proceeds.