Malcolm Turnbull has long admired John Key and said so many times, and the feeling is mutual.

A month after Turnbull reclaimed the Liberal leadership from Tony Abbott and became Prime Minister of Australia, John Key was the first leader on his overseas dance-card.

That was in October last year. Key declared his admiration for Turnbull and said he believed he would be a long- serving Prime Minister.

Between then and now, the pair have met on no fewer than four occasions, including at Apec in the Philippines where Mr Turnbull and President Barack Obama were caught on a hot mike talking about what a great guy John Key was.

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The other recent meetings have been at the East Asia Summit in Kuala Lumpur; at the Commonwealth heads of Government meeting in Malta and at climate change talks in Paris.

The pair have a lot in common; they are both on the socially liberal wing of Conservative ruling parties. They both support gay marriage.

They are both wealthy and they were both merchant bankers, Mr Turnbull with Goldman Sachs and Mr Key with Merrill Lynch.

They both had the confidence to challenge sitting MPs for their seats, Mr Key in 2002 and Mr Turnbull in 2004.

However, they hold different views on the monarchy. Mr Key is a devoted monarchist whereas Mr Turnbull headed Australia's republican movement for many years.

Mr Turnbull was a journalist then lawyer before his banking career. His wife, Lucy, is a former Lord Mayor of Sydney.