This makes him ineligible to stand for Parliament as only sole-Australian citizens may do so.

Dunne's office told Stuff that the Department of Internal Affairs (DIA) looked into the matter after receiving questions from Fairfax Media Australia last week.

They received legal advice that Joyce was a citizen by descent, as his father was born in New Zealand. DIA then had this checked by Crown Law, and then conveyed this advice to Dunne and the Ministry of Foreign Affairs and Trade on Wednesday.

However DIA is at this point not confirming this. Joyce has asked the Australian government to refer him to the High Court after he was asked a series of questions about whether he could be a dual citizen of New Zealand.

READ MORE: What Barnaby said when other pollies got caught out on citizenship

Joyce says he has advice from the Australian Government Solicitor that he is in the clear.

But the New Zealand High Commission has told him he might be a New Zealand citizen by descent.

Joyce says he was "shocked" to hear that.

In shock developments at Parliament House on Monday, 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.

The Deputy Prime Minister was born in Australia and has never applied for New Zealand citizenship

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 New England.

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

"Neither I, nor my parents have ever had any reason to believe I may be a citizen of another country. 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 Joyce could well be a dual citizen, potentially disqualifying him from Australian 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".

Joyce's father James Michael Joyce was born in 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. 

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 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 New Zealand 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 New Zealand 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 NZ citizen at birth courtesy of section 7 of the 1948 Act," Hood said.

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

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 an NZ citizen by descent if born to a Kiwi father.

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 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 Joyce's case resembles that of fellow Nationals MP Matt Canavan, who was apparently given Italian citizenship by descent after his mother made an application on his behalf. The High Court judgement on Senator Canavan's case could affect 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."

Comment was sought from the NZ 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 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," Windsor, a long-time foe of the Nationals leader, said.

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

smh.com.au