New Zealanders, like Australians, have been highly amused by the constitutional problems of their Deputy Prime Minister, Barnaby Joyce, on discovering he has our dual citizenship.

But there is a serious lesson here about written constitutions.

Under Section 44 of the Australian Constitution dual has been interpreted as "allegiance to a foreign power", which makes an Australian ineligible for Parliament.

But the same provision has previously unseated a member of the Senate who held British as well as Australian citizenship, and this year three senators, Scott Ludlam ("New Zealander"), Larissa Waters ("Canadian") and Matthew Canavan ("Italian") were forced to resign.

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All were simply Australian as far as they knew.

Ludlam's family had moved to Australia when he was eight and he thought he had given up New Zealand citizenship when he took out Australian citizenship in his mid-teens.

Waters' Australian parents were briefly in Canada when she was born, and Canavan, unbeknown to him, had been registered by his mother as an Italian resident in Brisbane.

Now Joyce, born in Australia, fiercely Australian, has learned he inherited New Zealand citizenship from his father. He was not amused and nor were the rest of the Liberal-National coalition government, which would lose its majority if the High Court rules he cannot retain his seat.

They must be cursing New Zealand's generous citizenship rules that extend its privileges to children born overseas to New Zealand citizens. Australia, as New Zealanders know too well, is much more jealous of its privileges, denying some of them even to residents who have spent their lives in Australia if their parents did not have citizenship.

Federal ministers were probably blaming New Zealand for their plight even before they realised the Labor Opposition had received some help from a member of their fraternal party in New Zealand.

All parliamentarians ought to have a working knowledge of diplomacy. New Zealand Labour MP Chris Hipkins ought to have known better than to ask a question of Internal Affairs Minister Peter Dunne that could have such repercussions in Australia.

Dunne was already dealing with questions from Australian journalists who were aware of Joyce's potential problem.

It is hard to believe Hipkins did not know he was getting involved in mischief against the Government in Canberra. It is hard to blame Foreign Minister Julie Bishop for warning she would find it "very hard to build trust" a Labour Government in New Zealand if one emerges from our election next month.

Hipkins put his new leader, Jacinda Ardern, to an early test of her own diplomacy. She has passed the test quite well, admitting Hipkins' fault but also upbraiding Ms Bishop for rushing to judgment and taking steps to preserve a good relationship.

Now that the dust has settled, New Zealanders can reflect on how fortunate they are not to have a written constitution.

To be barred from Parliament by dual citizenship a candidate has never sought, never even known they possessed, is the sort of unforeseen implication of immutable law that results in absurdity.