New Zealand's Prime Minister has confirmed that "unwittingly or not", Barnaby Joyce is a citizen of the country because his father was born there.

Key points: Barnaby Joyce revealed "shock" over NZ citizenship doubts

Barnaby Joyce revealed "shock" over NZ citizenship doubts NZ says as his father was born there, Joyce is automatically a citizen

NZ says as his father was born there, Joyce is automatically a citizen High Court to rule on Joyce's eligibility for Parliament

The Deputy Prime Minister revealed on Monday morning that he was contacted by the New Zealand High Commission last Thursday and advised he "could be a citizen of New Zealand by descent".

Prime Minister Bill English said he was informed about Mr Joyce's case late last week and according to local law, he is a New Zealand citizen.

"It is a matter for the Australian system to decide how Australian law applies in his case and how they deal with this issue," he told reporters.

Analysis by Antony Green [In a 1992 ruling] the High Court said just because a country gives an Australian citizenship doesn't mean that Australia has to recognise it. Out of that judgement came this view that if you've got a foreign citizenship, you must make all reasonable steps to get rid of it. In that original case, Justice Deane spoke about the need for a candidate having to accept or acquiesce in having this foreign citizenship. You would think that those words would come into play, particularly in this case. That's why the Government is quite confident about Barnaby Joyce. If the High Court says he's disqualified, probably 40 per cent to 50 per cent of Australians are disqualified from running for parliament. How can you have adherence to a foreign power if you don't know you have it? Clearly the Parliament is throwing a number of cases at the court in the hope the court will provide greater clarity on what is meant by Section 44(1).

Under New Zealand law, children born to New Zealand citizens automatically obtain citizenship. It does not have to be activated.

It will now be up to the High Court to decide whether Mr Joyce, in holding dual citizenship, has breached Section 44 of the constitution and is eligible to sit in Parliament.

Mr English appeared to have some sympathy for Mr Joyce, saying cases like his are "almost always accidental".

"No-one sets out to confuse the public with their citizenship," he said.

Labor demands Joyce stand aside

But with his Government's slim, one-seat majority in the House of Representatives potentially hanging in the balance, Mr Joyce's political opponents can smell blood.

Both the Greens — which have lost two senators over dual citizenship dramas — and the Labor Party have called on Mr Joyce to resign as Deputy Prime Minister until the High Court rules on his case.

"How on Earth can this Government have someone in the office of Deputy Prime Minister when they don't even know if he's meant to be a Member of Parliament?" Labor frontbencher Tony Burke asked.

He pointed out that Nationals senator Matt Canavan recently resigned from Cabinet after learning that he had unwittingly become an Italian dual citizen at the age of 25 when his mother registered him with Italian authorities.

However, Mr Joyce is refusing to stand down, saying the Government's lawyers were of the "firm view" that the High Court would not disqualify him because his New Zealand citizenship had been conferred on him without his knowledge.

Turnbull 'very confident' Joyce won't be disqualified

Malcolm Turnbull had told Question Time he was confident Barnaby Joyce would not be disqualified ( AAP: Lukas Coch )

Prime Minister Malcolm Turnbull strongly defended his deputy during a rowdy Question Time, saying the Government was "very confident" of its legal advice and predicting the High Court will rule in Mr Joyce's favour.

"The leader of the National Party, the Deputy Prime Minister, is qualified to sit in this House and the High Court will so hold," he said.

In the face of Labor's assault Leader of the House Christopher Pyne warned the Opposition against "opening Pandora's Box".

He named four MPs whose citizenship could also be under a cloud — Susan Lamb, Tony Zappia, Justine Keay and Maria Vamvakinou.

Earlier, Opposition Leader Bill Shorten "politely declined" Mr Turnbull's suggestion of a joint referral to the High Court of any federal politicians "whose circumstances may raise questions under Section 44".

Labor's national secretary Noah Carroll also released a statement outlining the party's exhaustive vetting process, and said he was "confident every member of the Labor caucus has been properly elected".

Mr Joyce said he was "shocked" when he received the news from the New Zealand High Commission, adding: "I have always been an Australian citizen."

The National Party leader was born in Tamworth in New South Wales in 1967.

He said his father was born in New Zealand but moved to Australia in 1947 "as a British subject — in fact we were all British subjects at that time".

"Neither my parents nor I have ever applied to register me as a New Zealand citizen, the New Zealand Government has no register recognising me as an New Zealand citizen," Mr Joyce told Parliament.