Fiji's state-funded Legal Aid Commission is trying to overturn the convictions of more than 100 soldiers jailed for their involvement in the coup and mutinies of 2000.
Radio Fiji reports that the director of Legal Aid, Makareta Waqavonovono, is arguing before the High Court that the military commander, Commodore Bainimarama, has no powers to convene a court martial.
This is because in Britain the law has been changed to allow independent authorities to convene courts martial, not the military.
Ms Waqavonovono says since the Fiji Army Act is based on the British Army Act, any amendments there should apply to Fiji, otherwise the convicted soldiers' constitutional rights would be breached.
But the military lawyers have told the High Court that British law amendments haven't been binding in Fiji, since independence in 1970.
Judge Gerald Winters has adjourned the case until Monday to allow more time for legal arguments.