Now the High Court has ruled letters between the Governor-General Sir John Kerr and the Queen sent around the time of Prime Minister Gough Whitlam's dismissal should be released.

It's the culmination of a decade-long fight by Whitlam's biographer, Monash University Emeritus Professor Jenny Hocking to gain access to the correspondence, which she believes will shed light on the events of 1975 and potentially have implications for royal secrecy in other Commonwealth nations.

"We know very little in terms of what transpired between the Governor-General and the Queen at that time," Professor Hocking said.

"We know from Kerr's papers he was in very close contact with the Queen's private secretary Sir Martin Charteris, and refers to Charteris' advice about the dismissal.

"There's no doubt these letters are really critical to understanding how extensive that contact with the Palace was and what aspect the contact actually played in Kerr's thinking at the time."

Republicans are also hoping the case spurs interest in the stagnating push for Australia to break away from the monarchy.

Professor Hocking is seeking to meet with National Archives of Australia director-general David Fricker next week about the release of the letters.

"I'm expecting the archives to have these letters read when I go into the archives next week," she said.

In a statement, Mr Fricker said the archives accepted the ruling and would now get to work preparing the records for release.

He claimed the archives were a "pro-disclosure organisation" and wholly released 96 per cent of requested records last financial year.

The archive holds more than 200 of these letters and telegrams sent between Kerr and the Queen.

The letters are said to mostly address "topics relating to the official duties and responsibilities of the Governor-General" as well as reports about events of the day in Australia.

However, Kerr was anything but a typical Governor-General. He made history for dismissing the Whitlam as Prime Minister and installing Malcolm Fraser as caretaker, after the Opposition used its control of the Senate to block supply.

The letters were handed to the archives by Kerr's official secretary David Smith in 1978 after Kerr's retirement in 1977 under a letter of deposit, stating that the package contained "personal and confidential correspondence".

"In accordance with The Queen's wishes and Sir John Kerr's instructions, these papers are to remain closed until 60 years after the end of his appointment as Governor-General, i.e. until after 8 December 2037," the letter stated.

"Thereafter the documents are subject to a further caveat that their release after 60 years should be only after consultation with the Sovereign's Private Secretary of the day and with the Governor-General's Official Secretary of the day."

However, these conditions were changed after Kerr's death by the Queen herself. While the letters could be released as early as 2027, they could only be made public with the express approval of the Governor-General's official secretary and the Queen's private secretary's approval.

This amounted to an indefinite "Queen's embargo", Professor Hocking said, and was unusual in that it meant Australian laws about the management of our documents could not be applied to the letters.

Professor Hocking tried unsuccessfully several times to access the letters through the National Archives and through a freedom of information request to the Office of the Governor-General.

Backed by a crowdfunding campaign and a team of high-flying pro bono lawyers, Professor Hocking lodged action against the National Archives in the Federal Court in December 2016.

Buckingham Palace, through correspondence with Government House in Australia that was submitted to the court, maintained that the letters should not be released.

In March 2018, Justice John Griffiths ruled the letters were "personal and confidential" and not Commonwealth records.

Professor Hocking lodged an appeal a month later, led by prominent Sydney barrister Bret Walker S.C.

A majority of the High Court on Friday set that ruling aside, and held that the letters should be considered Commonwealth records because it was the property of a Commonwealth institution, namely the official establishment of the Governor-General.

The fact the letters were deposited in the archives by Smith in his capacity as the Governor-General's Official Secretary strengthened the case, the High Court said.

"The nature and significance of the correspondence was such that it was only to be expected that the correspondence would be kept within the official establishment of the Governor-General as the functional unit of government responsible for keeping records created or obtained by the Governor-General in his or her official capacity," the ruling reads.

"With respect to the majority in the Full Court, we cannot see how the correspondence could appropriately be described, however 'loosely', as 'private or personal records of the Governor-General' even allowing for the ambiguity of the description of 'private or personal'.

"It cannot be supposed, for example, that Sir John Kerr could have taken the correspondence from the custody of the official establishment and destroyed it or sold it, and the sequence of events which resulted in its deposit with the Australian Archives demonstrates that such a possibility was never contemplated."

Professor Hocking is expecting some bombshells to be in there, given what she has uncovered in Kerr's papers in the past.

Most notably, Professor Hocking discovered the hitherto unknown role of former High Court Chief Justice Sir Anthony Mason in the sacking of Whitlam.

Kerr's records indicate Mason largely wrote the statement Kerr read while dismissing the Whitlam government.

"From what we already know, [the letters will have]have a very interesting story to tell and fill a critical gap in the literature," Professor Hocking said.

"History will be revised in that light."

The case may also lead to the release of other Governor-Generals' documents and have implications for state archival holdings, which have claimed an exemption for royal records in the past.

There could even be momentum across other Commonwealth nations to release similar records, Professor Hocking said.

"In terms of royal secrecy, the impact could be really significant," Professor Hocking said.

Republicans are also hoping the case reignites interest in Australia's relationship with the monarchy.

"In this case it is patently absurd that to this point we Australian's haven't had the right to know how much a member of the Royal family, unelected by the Australian people, did or didn't have to do with the dismissal of a democratically elected Australian government," Australian Republic Movement chair Peter FitzSimons said.