The evidence against a Sydney youth accused of planning a "terrifying" public terrorist act consists of a single intercepted phone call that was mistranslated by police, a court has heard.

And the defence barrister for the alleged terrorist plotter says his client has been placed in a high-security cell in the state's toughest prison at Goulburn "without explanation", and denied visits from his family.

Omarjan Azari following his September arrest. Credit:Daniel Munoz

Omarjan Azari, 22, was arrested at his home at Guildford in Sydney's west in September during the largest counter-terrorism operation in Australia's history.

He was charged with conspiring to plan or prepare a terrorist act, which police allege was intended to "shock, horrify and terrify the community".