Mr Keating’s comments come just days before Prince Charles arrives in Australia for the opening of the Commonwealth Games on the Gold Coast.
Mr Keating told the British press he was certain Prince Charles, who he met several times during his prime ministership, would support a republic.
“I have no doubt he believes Australia should be free of the British monarchy and that it should make its own way in the world,” Mr Keating told the Sunday Times in Britain, which passed the comments onto The Australian.
“Why would he or any one of his family want to visit Australia pretending to be, or representing its aspirations as, its head of state?
“But none of that is to diminish the commitment and sense of duty that Prince Charles displays towards Great Britain and, as constitutional arrangements stand, towards Australia.
“He is a great friend of Australia — there is no doubt about that.”
The Prince of Wales and the Duchess of Cornwall, Camilla, will arrive in Australia on April 4 for a royal visit announced by prime minister Malcolm Turnbull in February.
They will visit Brisbane, then the Commonwealth Games on the Gold Coast.
Mr Keating has long supported a republic.
In 1999, Australia held a referendum on replacing the Queen with an Australian head of state, but the No side emerged victorious.
Mr Keating discussed the republic with Prince Charles back in 1993 on a visit to Scotland.
In 1994, Prince Charles said the emerging debate in Australia was a sign of a “mature and self-confident nation”.
Mr Turnbull himself supports a republic and campaigned for the Yes vote in the 90s.
Since becoming prime minister he has reaffirmed his support, but also described himself as a loyal “Elizabethan”.
Former prime minister Tony Abbott, a firm monarchist, took to Twitter to criticise Mr Keating's comments.
"Prince Charles would just want to do his duty and he shouldn't be verballed by an ex-PM," Mr Abbott wrote.
Another senior Liberal, Eric Abetz, also rounded on Mr Keating, accusing him of mounting "self-serving and egotistical" arguments for a republic.
"Mr Keating's musings clearly have no basis in fact and are just a sad feature of the latest Republican push," the Tasmanian senator said in a statement.
