Australia's prime minister has cast doubt over whether Prince Charles will be head of state of the Commonwealth country.

Malcolm Turnbull raised the issue of removing the monarchy and becoming a republic after Queen Elizabeth.

"I don't think it will become a frontline issue before then," Turnbull said in Sydney on New Year's Day.

In a 1999 referendum, 54.87% of Australians voted to keep the Queen as the head of state. Turnbull, who had campaigned for a republic, added: "We all say 'long live the Queen' with great sincerity and love."

If the issue is reignited, Liberal Party leader Turnbull said the first thing needed "is an open discussion about how a president would be elected".

According to The Mirror, the prime minister suggested holding a postal vote on the issue.

Turnbull's remarks come on the heels of criticism by Australia's former Prime Minister Paul Keating, who blasted Turnbull for not pushing the issue of forming a republic. "How utterly absurd that we have to borrow the monarch of another country," Keating said. "How pathetic."

Before Liberal Party leader Turnbull met the Queen at Buckingham Palace last July, he said: "Most Australian republicans are Elizabethans as well."

The Queen is head of state of 15 Commonwealth Realms, including Barbados, Canada, Jamaica and New Zealand. The Mirror noted that many of those countries could rethink their ties to the Crown when Charles becomes king.

So far, Barbados and Jamaica have announced their intention to replace the Queen but have yet to do so.