John Key resigns: Where now for the NZ-Australian political bromance?

Australia's Prime Minister Malcolm Turnbull said his response to the resignation of his New Zealand counterpart was a text message: "Say it ain't so, bro."

After sharing kayaking trips around Sydney harbour in February and late-night summit dinners, the political bromance between the two prime ministers is coming to an end.

John Key's surprise resignation may have caught the Kiwi media off guard, but his opposite number across the Tasman Sea claims to have known what was coming.

"I have to say his announcement didn't come as a complete surprise to me," Malcolm Turnbull told the media.

"He has gone," he said, "when he is on top. He is a class act, none classier."

Profile: From banker to three-term PM

Resignation 'changes the political game'

The praise heaped on Mr Key by his Canberra counterpart reflects the political, centre-right bond the two men share.

Both have a background in business and finance, and both have sought to bring those deal-making skills to the political arena.

But Mr Turnbull's words may reveal a little envy too.

While Mr Key gets to decide when to draw his eight-year term to an end, Australian politics has a far more frenetic pace, having chewed its way through four prime ministers since 2008.

Mr Turnbull is currently suffering poor approval ratings and holds the slimmest possible parliamentary majority, which means much of his time is spent answering questions about possible leadership challenges.

Conversely, Mr Key's position seemed rock-solid, with his relative longevity in the post making him more popular.

"He has undertaken big reforms in New Zealand," Mr Turnbull noted, "but he has been able to do so and maintain strong popular support".

Very few politicians get to choose the timing of their exit in this way, something Malcolm Turnbull clearly appreciates.

But like many relationships, this one will end with some unfinished business.

John Key was one of the architects of the Trans-Pacific Partnership (TPP) - a deal which, it was claimed, could bring huge benefits to Australia.

It is significant that one of the biggest trade agreements ever was signed in Auckland, with Mr Key hosting other world leaders.

US President-elect Donald Trump's rejection of the TPP means that part of his legacy has collapsed in the last few weeks.

It will be left to New Zealand's new leader to work out with the other signatories if the TPP can be of any benefit without US involvement.

In his resignation speech, Mr Key said his wife Bronagh had spent many nights and weekends alone without him.

In resigning, he is returning to her side. But he may still get the occasional call from his Canberra "bro", asking for advice on how to hold on to power.