A HISTORIAN has lost her court bid to force Australian authorities to release secret letters that would reveal what the Queen knew of her representative's scheme to dismiss the country's government more than 40 years ago.

The National Archives of Australia has categorised the correspondence between the monarch, who is also Australia's constitutional head of state, and her Australian representative, governor-general Sir John Kerr, as "personal" and it might therefore never be made public.

The Federal Court on Friday agreed that the letters were "personal" and not state records, dismissing Monash University historian Jenny Hocking's application to make them public.

The letters would disclose what, if anything, the Queen knew of Kerr's plan to dismiss prime minister Gough Whitlam's government in 1975.

Hocking, who wrote an acclaimed biography of Whitlam, has not ruled out an appeal.

She described the ruling as "a disappointing decision for our history".

Justice John Griffiths acknowledged in his judgment a legitimate public interest in the letters "which relate to one of the most controversial and tumultuous events in the modern history of the nation".

The fall of Whitlam's government is the only time in Australia's history a democratically elected federal government was dismissed on the British monarch's authority.

Kerr's surprise intervention placed unprecedented strain on Australia's democracy and bolstered calls for the nation to become a republic.

Hocking had argued the letters should be released regardless of the Queen's wishes because Australians have a right to know their own history.

Without the "personal" classification, the letters could have become public 30 years after they were written like other government documents held in the Archives.

Under an agreement struck months before Kerr resigned in 1978, the letters covering three tumultuous years of Australian politics will remain secret until 2027. The private secretaries of both the sovereign and the governor-general in 2027 still could veto their release indefinitely.