"I suspect the advice that would have been given to him (Sir John) was that it would have been prudent to hold off a little bit longer," Sir William Heseltine told The Australian.

"But obviously he felt the pressure of these two contingencies about the election and the financial situation were too pressing to ignore.

"I think it was very proper of him not to ask, and in ways which are now very evident, very sensible and satisfactory that he didn't."

Sir William was the Queen's assistant private secretary in 1975, when Sir John sacked then prime minister Mr Whitlam.

"All of us in the office in London thought that if Kerr had been able to hold his nerve for just a day or two more, there probably would have been a political solution to the problem, which would have avoided a lot of fuss," Sir William said.

"I think various possibilities would have eventuated. Somebody in the Senate would have given way ... and the vote (on supply) could have been passed in the Senate. That's one thing I think might have happened."

Previously secret correspondence between the governor-general and Buckingham Palace was released by the National Archives on Tuesday.

The letters revealed Sir John deliberately chose not to inform the Queen of his decision to sack Mr Whitlam ahead of time.

However in the week before Mr Whitlam's November 11, 1975 dismissal, the Queen's private secretary Sir Martin Charteris discussed with Sir John the powers the governor-general had available to him.

Australian Associated Press