‘Like a sister’: Australia will miss Jacinda Ardern but trans-Tasman ties likely to stay strong

Jacinda Ardern is a "sister" of Australia and her resignation as prime minister is being lamented by the Albanese government, but her stepping down is unlikely to materially affect New Zealand's relationship with its trans-Tasman neighbour.

But experts believe there could be minor changes in the relationship later this year, when Ardern's Labour could lose to the centre-right National party in New Zealand's national election.

"The Australia-New Zealand relationship is very close, and will remain so irrespective of who the prime ministers are," said Allan Behm, a former diplomat who worked closely with New Zealand.

"But when you have PMs who get on really well, like John Howard and [former New Zealand prime minister] Jenny Shipley did, or clearly as Anthony Albanese gets on with Ardern, it makes the relationship that much more effective."

Ardern on Thursday announced she would step down as prime minister by 7 February, handing over to a successor ahead of a national election in October. She said she "no longer [had] enough in the tank" and wanted to spend more time with her family.

The Labour party is trailing in polls and is facing immense pressure and criticism over cost of living challenges, according to Robert Ayson, a professor of strategic studies at Victoria University of Wellington.

"The likelihood is National would have come into office in October even if [Ardern] stayed. The current government [is] getting pummelled in the polls with inflation and interest rates - the shine has definitely worn off," he said.

Since coming to office in 2017, Ardern has worked alongside three Australian prime ministers - Malcolm Turnbull, Scott Morrison and Albanese - with varying levels of warmness.

In an infamously uncomfortable press conference with Morrison in 2020, the pair disagreed about controversial migration laws that allowed for criminals with New Zealand citizenship to be deported from Australia.

Ardern's relationship with the Albanese government has been noticeably warmer.

Ardern said on her first visit to meet Albanese that there had been "points of friction" and "tension" in the two countries' relationship, a pointed reference to Morrison and the migration issue.

Changes late last year to migration rules for Kiwis, helping to fast-track permanent residency applications, have gone some way to assuaging the concerns raised by Ardern. Albanese previously committed to take a more "commonsense" approach when cancelling New Zealand citizens' visas.

In an interview with 2Day FM on Monday, Albanese listed Ardern as one of his international "besties".

And in a tweet after her announcement on Thursday, Albanese described Ardern as "an inspiration to so many and a great friend to me", saying she had "demonstrated that empathy and insight are powerful leadership qualities".

The treasurer, Jim Chalmers, said she was "more than a friend to Australia, she's more like a sister".

Behm, the former diplomat, and Victoria University's Ayson both said the history of the Australia-New Zealand relationship meant ties would remain strong, no matter who sat in the respective prime ministerial suites.

"She's had good rapport with Albanese in particular, it's been an easier relationship for New Zealand to manage than with the Morrison government, but even that was still in overall good shape, despite some strains," Ayson said.

Behm, now the head of the international and security affairs program at the Australia Institute, doubted even a potential change of government would shift the relationship.

Ayson said he expected a change in government would lead to "more continuity than change" in the relationship with Australia, and predicted a Christopher Luxon National government may be less likely to publicly agitate Canberra to alter migration rules.

"I don't think National want to push that relationship. Even on migration, I think they would be not inclined as much to raise that in the same vocal way Ardern did," he said.

"It's not the same issue it once was, with some changes on the Australia side."

Behm said the more immediate impact may be simply that the Australian government is saying farewell to a close neighbour that many senior ministers regarded as a personal friend and ally.

"I think under this government, we will miss Jacinda. She espouses so many values our government does," he said.