Barnaby Joyce tells parliament he may be dual national

In shock developments at Parliament House on Monday, Mr Joyce became the latest potential victim of a worsening citizenship saga after extensive investigations revealed he inherited dual citizenship through his father, who was born in New Zealand.

Questions over dual citizenship - which is prohibited for members of parliament under the constitution - have already forced two Greens senators to quit, Nationals senator Matt Canavan to resign as resources minister, and put question marks over a number of other MPs.

The Deputy Prime Minister, who was born in Australia and has never applied for New Zealand citizenship, said he was "shocked" to receive Fairfax Media's questions on Thursday. Fairfax Media also sent an additional request for comment about an hour before Mr Joyce's bombshell announcement.

He told Parliament he had decided to refer himself to the High Court for clarification but would not resign from cabinet and would remain as the member for the NSW electorate of New England.

"I was born in Tamworth, just as my mother and my great-grandma was born there 100 years earlier," he said.

“I was born in Australia in 1967 to an Australian mother and think I am fifth-generation. My father was born in New Zealand and came to Australia in 1947 as a British subject. In fact we were all British subjects at that time.

"The government has taken legal advice from the solicitor-general. On the basis of the solicitor-general's advice, the government is of the firm view that I would not be found to be disqualified by the operation of section 44 of the constitution from serving as the member of New England. However, to provide clarification to this very important area of the law, for this and future parliaments, I have asked the government to refer the matter.

"Given the strength of the legal advice the government has received, the Prime Minister has asked that I remain as Deputy Prime Minister and continue my ministerial duties."

But experts believe Mr Joyce could well be a dual citizen, potentially disqualifying him from Parliament and putting the Turnbull government's slim majority at risk.

Under New Zealand law anyone born overseas to a New Zealand father between 1949 and 1978 is considered a "citizen by descent".

Mr Joyce's father James Michael Joyce was born in the NZ city of Dunedin in January 1924, according to Australian Securities and Investment Commission company records. He moved to Sydney to attend university, later married and settled in NSW. Barnaby Joyce was born in Tamworth in 1967.

Mr Joyce has not been through any formal renunciation process and his office cannot say whether his father - now in his 90s - ever renounced. While the older Mr Joyce was born a British subject, it's believed he became a New Zealand citizen with the passage of a 1948 citizenship law.

Crucially, experts say NZ law - section 7 of the British Nationality and New Zealand Citizenship Act - confers citizenship by descent automatically, and does not need to be activated.

While there is a process for registering NZ citizenship by descent in order to attain a passport, University of Auckland international law expert Anna Hood believes that may not matter.

"If Barnaby's father was indeed a New Zealand citizen in 1967 then it looks like Barnaby may have become a New Zealand citizen at birth courtesy of section 7 of the 1948 Act," Dr Hood told Fairfax Media.

"There is a question about whether Barnaby or his parents would have had to register him as a New Zealand citizen. However, section 7 of the 1948 act does not explicitly require individuals born to New Zealand fathers to register their citizenship."

Dr Hood also points to another section of the act, which provides that every person born outside New Zealand between 1 January 1949 and 1 January 1978 shall be a New Zealand citizen by descent if born to a Kiwi father.

My Joyce's spokesman has previously said he independently established he was not a dual citizen "many years prior to entering Parliament". However his office has not elaborated on what steps he took.

University of NSW constitutional law expert George Williams said Mr Joyce could be "in some difficulty".

"The only question here is he regarded under New Zealand law as a citizen? That's not a question of activation - in fact this can happen without a person's knowledge or consent," Professor Williams said.

"It doesn't even have to be registered, it's just is he entitled to the benefits under their law should he wish to take them up? It doesn't matter if he has. If he has that entitlement, that is what the constitution extends to."

Anne Twomey from the University of Sydney believed Mr Joyce's case resembles that of Senator Canavan, who was apparently given Italian citizenship by descent after his mother made an application on his behalf. The High Court judgement on the case of Senator Canavan - who has stood down from the ministery but remains in the upper house - could affect Mr Joyce.

"The fact that another country can have these rules about citizenship by descent, which people have not the foggiest idea about, is potentially problematic. It may be that the High Court tries to wind these things back – so if you were born in Australia, and you haven't taken any steps to gain any foreign citizenship, then maybe that is enough to avoid disqualification," she said.

"We need to know whether the mere fact that you are descended from someone and that gives you the right to do something in relation to foreign citizenship – like obtaining a passport – is sufficient to trigger this or not? I am sure many politicians would like to know that."

Senator Canavan was forced to step down, after it was revealed his mother had applied for Italian citizenship on his behalf, when he was 25.

But he is asking the High Court to rule whether he is still eligible to stand in Parliament, as he was unaware his mother had taken this step.

When Senator Canavan stepped down from the cabinet, Mr Joyce took on his ministerial responsibilities. 

Mr Joyce, who has stood by Senator Canavan, never applied for citizenship, nor did his family.  Under a quirk of New Zealand law, Mr Joyce automatically inherited citizenship from his father. 

The High Court considers whether MPs took all reasonable steps to renounce their citizenship.

Mr Joyce, who is not registered with New Zealand authorities as a citizen, is expected to argue he had no way of knowing he was a dual-citizen, having previously inquired to see if he was registered with New Zealand authorities, and receiving a 'no'.

Fairfax Media sought comment from the New Zealand Department of Internal Affairs last week, but they did not respond.

But on its official website, the department noted: "If you were born overseas and at least one of your parents is a New Zealand citizen by birth or grant, you are an NZ citizen by descent."

In separate education material, the department says: "Like a citizen by birth, a New Zealand citizen by descent acquires his or her citizenship status automatically at the time of his or her birth. However many people choose to register their citizenship by descent status with the Citizenship Office of the Department of Internal Affairs in order to obtain a certificate to prove they are a New Zealand citizen."

And in a 2015 press release, the department said: "Babies born in Australia and other countries to New Zealand parents are still Kiwis. Before an overseas based family brings their new baby home to meet the Kiwi cousins, they need to register him/her as a citizen by descent before they can apply for a New Zealand passport."

The registration process involves filling out a form labelled "Confirmation of New Zealand Citizenship By Descent", rather than calling it an application for citizenship. The registration process costs NZ$204.40.

Former independent MP Tony Windsor, who challenged Mr Joyce for his seat of New England at the 2016 election, wouldn't rule out another attempt if the citizenship fiasco triggered a by-election.

"It's not front of mind but you never rule anything out in life," Mr Windsor, a long-time foe of the Nationals leader, told Fairfax Media.

"If he's ruled out, it's a whole new ball game."

Prime Minister Malcolm Turnbull wrote to Opposition Leader Bill Shorten on Monday, advising him of Mr Joyce's referral to the High Court, and offered him the "opportunity" to refer any Labor MPs who may potentially fall foul of the same constitutional section.

"It is manifestly in the national interest that the High Court have the opportunity to clarify the limits on the operation of section 44 of the Constitution," Mr Turnbull wrote.

"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.

"…I a writing to you to offer you the opportunity to nominate any Labor members of senators whose circumstances may raise questions under section 44 of the Constitution so that the Parliament can also refer these matters to the High Court for its consideration.

"The Australian people must have confidence in our political system and resolving any uncertainty is vital."

Mr Turnbull said the legal advice sought by the government left it "satisfied that the Court would not find Mr Joyce disqualified to sit in the House".