The constitutional citizenship headaches have extended to the major parties and now right up to the Deputy Prime Minister.

Labor and the Coalition had been confident they would avoid the humiliation the Greens faced as their two deputy leaders — Scott Ludlam and Larissa Waters — quit after finding out they held dual citizenship.

But it has grown to be beyond a crisis for the Greens, cutting a swathe through the Nationals and becoming a test of the Government's hold on power.

Citizenship extends through generations

One Cabinet minister, Matt Canavan, joined the list of those with a complication over section 44 of the Australian constitution last month but now the citizenship uncertainty has proliferated to hit Barnaby Joyce.

The New Zealand Government says Mr Joyce has dual citizenship because his father was born there.

And in another blow to the junior Coalition partner, now the deputy leader is also being referred to the High Court because she has dual citizenship through her Scottish-born father.

Senator Nash said her parents told her she was not a dual citizen but it was only when Mr Joyce announced his Kiwi citizenship that she checked — her circumstances match those of her leader.

He had thought he was only an Australian citizen because he had never applied or registered to be a New Zealander.

But the intricacies of citizenship laws in New Zealand have caught him out too.

When the citizenship crisis hit the Greens, the big political operations thought they would be safe because they try to inspect every element of a candidate's history to try to avoid these sort of embarrassments.

But even that level of intense inspection could not have picked up that Senator Canavan's mum had apparently registered him as Italian without telling him.

Mr Joyce's inherited New Zealand citizenship even though his father moved to Australia before NZ even had its own citizenship status.

Senator Nash inherited her British citizenship from her father, who moved to Australia from Scotland.

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Those cases are a step removed from the Greens duo — who both knew they were born in another country and wrongly assumed they were still eligible.

This debacle has already prompted the Greens to rethink their more relaxed approach to scrutinising candidates, beginning with a closer examination of their birth certificates.

The Nationals have not announced any similar crackdown on their purity of their candidates' citizenship, but with both leader and deputy leader and their rising star Matt Canavan all heading to the High Court — it is all but certain they will.

It will be up to the Court to examine questions going back generations to determine if Senator Canavan, Mr Joyce and Senator Nash are still eligible.

The argument will be they could not hold allegiance to another nation when all thought the only citizenship they held was Australian.

Unlike Mr Joyce and Senator Canavan, the Greens are not challenging their more straightforward cases.

Millions of Australians have foreign heritage

Barnaby Joyce says he was "shocked" by the citizenship news. ( AAP: Lukas Coch )

With the latest census showing 28 per cent of Australians born overseas, and many more with a family history beyond that — the citizenship hurdle is one that many potential candidates have to clear.

One Nation Senator Malcolm Roberts is one of those born overseas — in India.

But he has also been referred to the High Court because of questions about whether he renounced British citizenship, acquired from his Welsh-born father, before he nominated for Parliament.

He has told other senators he filled in the official form months later — in November.

"With around half of all Australians having a foreign born parent, and with many foreign nations having citizenship laws which confer citizenship by descent regardless of place of birth, the potential for many, possibly millions of Australians unknowingly having dual citizenship is considerable," Prime Minister Malcolm Turnbull said.

With Mr Joyce, Senator Nash and Senator Canavan under question, the Coalition is putting pressure on Labor MPs to prove they are not dual citizens and has challenged ALP MPs born overseas to produce their documentation to show they have properly renounced any extra citizenship.

Do the due diligence

Dual citizenship is not a barrier for entry to state politics, but the path to Federal Parliament is clearly signposted with flashing lights alerting a political aspirant of the need to "determine your eligibility".

That advice is at the top of the list on the Electoral Commission's nomination guide for candidates, followed by detailed instructions to read the electoral backgrounder on constitutional disqualifications.

And when they pick up the pen to fill in the boxes on the nomination form, the reference to the eligibility section of the constitution is in bold.

They are asked to fill in whether they are an Australian citizen by birth — including where they were born, or if it was by naturalisation including when citizenship was granted.

That is the step that has tripped up the two Greens senators because they made assumptions and did not check before signing the documents.

In Mr Ludlam's case, he wrongly thought that naturalisation wiped out his New Zealand citizenship, while Ms Waters believed she was not a Canadian citizen.

Twenty-five people in the current Parliament were born overseas, including the outgoing Greens, making citizenship a nerve-wracking topic for many.

It has prompted suggestions that the rules should be changed so that a candidate has to produce the documentation to prove they have renounced citizenship of another country.

But there is no inclination for that step within the Government, where the argument is that an aspiring politician should treat the nomination form as a solemn and binding responsibility.

But in senior ranks of the Government there is a strong hope that the High Court will clarify the citizenship situation, particularly for those born in Australia but who also have citizenship of another nation because of their parent's heritage.

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