The Australian government accused a New Zealand opposition party on Wednesday of conniving to bring it down by revealing that Australia's deputy prime minister is a New Zealand citizen, and thus ineligible to sit in parliament.
The outcry over the deputy prime minister of a government with a one-seat majority has led to rare acrimony between the long-term allies, whose relations are usually defined by chummy banter and fierce sporting rivalry.
Australian Deputy Prime Minister Barnaby Joyce said on Monday he may not be eligible to sit in parliament after being told he was a dual citizen by descent.
The New Zealand government later said Joyce held New Zealand citizenship as his father was born there. His mother was Australian and his father came to Australia in 1947 as a British subject.
Australian politicians are not eligible to be elected to parliament if they hold dual or plural citizenship.
The revelation about Joyce's roots came to light after New Zealand's opposition Labour Party member Chris Hipkins raised the matter in parliament there.
Australia's government saw Hipkins' question as part of a dark plot between the opposition in New Zealand and Australia to "steal government".
"It is extraordinary that a New Zealand member of parliament has allowed himself to be used by a party in a different country with an intent to bring another party in that country down. It is quite extraordinary," Foreign Minister Julie Bishop told Parliament on Wednesday.
"This is highly unethical, at least. But, more importantly, puts at risk the relationship between the Australian government and the New Zealand government," Bishop said.
She said would find it hard to trust members of a political party that had been used to undermine the Australian government.
The leader of the New Zealand Labour Party, Jacinda Ardern, rejected Bishop's accusations as "false claims".
Australia and New Zealand see themselves as the closest of allies with common heritage, bonds of kinship and shared interests. Their troops have fought alongside each other in far-flung wars for generations, most recently in Iraq and Afghanistan.
The kerfuffle could play into a New Zealand general election on September 23, with the Labour Party enjoying a surge in support.
New Zealand Prime Minister Bill English said he accepted the Australian reaction, and he took the opportunity to give the Labour Party a ticking-off for getting mixed up in Australian politics.
"The Labour Party in New Zealand has to explain that because it's very poor judgment that they got involved with it," English told reporters in Wellington.
Australia's High Court will begin hearings into Joyce's eligibility on August 24. Should he be ruled ineligible, Prime Minister Malcolm Turnbull would be forced to rely on independents to get legislation passed.
Turnbull said he expected the court to rule in Joyce's favour but independents are smelling blood.
"I've been treated like a door mat, I am a door mat but in a couple of weeks, I may not be," independent MP Bob Katter told Reuters.