Australia's opposition Labor Party on Wednesday rejected extraordinary claims from the country's top diplomat that it conspired with New Zealand leftists to try to topple the government.
Labor's Penny Wong accused Foreign Minister Julie Bishop of "a Kiwis under-the-bed scare campaign" to divert attention from government woes, invoking anti-communist "Reds under the beds" fears of the Cold War.
"This sort of behaviour, I don't think is particularly good for democracy," Wong told reporters.
The international spat stems from Deputy Prime Minister Barnaby Joyce's revelation this week that he was a New Zealand citizen, meaning he may be forced to step down under rules barring dual nationals from sitting in Australia's parliament.
That would be a disaster for Prime Minister Malcolm Turnbull's conservative government, which only holds a one-seat majority.
Officials in Wellington said Joyce's status came to light after queries from Australian journalists.
However, New Zealand Labour MP Chris Hipkins also admitted asking questions about the issue last week after talks with an Australian acquaintance, since revealed as Wong's chief of staff Marcus Ganley.
Foreign Minister Julie Bishop said Ganley's involvement showed Wong was "up to her neck in it", along with Labor leader Bill Shorten.
- 'Act of treason?' -
Bishop stood by an allegation made Tuesday that the New Zealand and Australian opposition parties were trying to "undermine the government of Australia", even suggesting the plot could be global.
"I'd like to know if Bill Shorten is directing his troops to ask these questions in other parliaments around the world?" she told Sky News.
Wong conceded her staffer's actions were unwise but said they amounted to "a chat with a mate (Hipkins)" rather than a secretive attempt to bring down the government.
"That Ms Bishop can seriously suggest that there is some act of treason here is ridiculous," she said.
Wong said Bishop's "extraordinarily reckless and irresponsible" accusations risked straining relations with New Zealand, one of Australia's closest allies.
New Zealand's centre-right Prime Minister Bill English said Bishop's comments were made "in the heat of the moment".
"Look, I can understand why they'd take it seriously over there, she's in a government with a majority of one," he told reporters.
"But we wouldn't want to let those comments get in the way of a positive relationship."
Joyce, whose Kiwi citizenship sparked the row, backed Bishop for "calling out" clandestine schemers.
"Obviously there's concern if you're sort of working in the background and trying to do something clandestine that could affect our nation's government," he said.
"If there's a suspicion of that, you've got to be called out on it, and that's exactly what Julie did."
- 'Shellshocked' -
Australia-born Joyce renounced his New Zealand citizenship on Tuesday and has insisted he will stay on until the High Court determines his fate.
He has said he was "shellshocked" to discover last week that he had automatically acquired New Zealand citizenship through his Dunedin-born father.
Section 44 of Australia's constitution bans dual citizens from parliament on the basis that lawmakers should be loyal solely to the country in which they were elected.
It was a little-known provision until July, when Greens party's co-deputy leader Scott Ludlam resigned after revealing he had dual Australian-New Zealand citizenship.
Another Greens senator, Canadian-born Larissa Waters, soon followed, while Matt Canavan stepped down as the government's resources minister after finding his mother signed him up to Italian citizenship in his 20s.
Indian-born One Nation senator Malcolm Roberts is also facing questions about his citizenship.
Almost half of Australia's 24-million population was born overseas or have at least one foreign-born parent, according to last year's census.
The Sydney Morning Herald said in an editorial this week that the law should reflect "commonsense", arguing there could be no questioning the loyalty of politicians such as Joyce who never even knew they were dual nationals.