It comes after an Australian tabloid published claims that the late Tongan king and a former diplomat helped to conceal the killing of George Brown.
Australian police believe Mr Brown was tortured and murdered in 1984 by members of the horseracing community.
The case, which rocked the industry, has never been solved.
Mr Brown's body had been found in a burnt car in bushland south of Sydney, with signs that he had been assaulted before his death.
