Former prime minister Paul Keating has urged Malcolm Turnbull to save Australia from Prince Charles becoming head of state, calling the prospect "deeply sick".

In a conversation with ABC journalist Kerry O'Brien at the Sydney Opera House on Tuesday night, Mr Keating reiterated his call for Australia to become a republic.

"No great country has a monarch ... of another country as their head of state," he said.

"And no great country has a flag of another country in the corner of their flag."

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The former Labor prime minister said he expected Mr Turnbull to win the next election and said the issue should be addressed in his government's next term.

"It requires a prime minister to take it on," he said.

"You can have all the republican movements you like, but if a prime minister doesn't want to take this on, it won't happen."

He said the idea of ending up with Prince Charles as Australia's head of state after the reign of Queen Elizabeth II ended was "deeply sick".

"What are we going to end up with?" he asked.

"Charles and Camilla, for God's sake."

He said Mr Turnbull faced other problems within his party after wrestling the leadership from Tony Abbott.

"The real question is, can he take the now very right-wing Liberal Party anywhere back near the centre?" he asked.

"That'll be the real test ... or whether he's stuck with the Loony Tunes show on the right."