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

The National Archives of Australia has categorized the correspondence between the British 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 has not ruled out an appeal.