New Zealand authorities have confirmed Australia's Deputy Prime Minister Barnaby Joyce is a Kiwi citizen, potentially rendering him ineligible to sit in parliament.

Mr Joyce told Australia's parliament on Monday the New Zealand High Commission contacted him last Thursday with the "shocking" information that he could be a New Zealander by descent.

Mr Joyce's father was born in New Zealand.

"On the basis of preliminary advice from their Department of Internal Affairs - which had received inquiries from the New Zealand Labour Party - they considered I may be a citizen by descent of New Zealand," Mr Joyce said.

A spokesman for Internal Affairs Minister Peter Dunne said by virtue of his father's citizenship Mr Joyce is a New Zealand citizen, whether he knew it or not.

Mr Joyce has said neither he nor his parents registered him for New Zealand citizenship, but Mr Dunne's office confirmed it was automatically conferred by descent.

"It has been confirmed by Crown Law - they have had a look at it - yes he is," the spokesman told NZ Newswire.

Prime Minister Bill English said he was notified last week that Mr Joyce's citizenship "could be an issue".

"(DIA) say that unwittingly or not he's a New Zealand citizen," he said.

Mr English said he wasn't aware of any other New Zealand citizens currently in Australia's parliament, but said he expected some would be looking very closely at whether they have Kiwi parents.

He joked that Mr Joyce, a fellow National Party leader, might fit nicely into New Zealand politics but said that probably wasn't on his mind at the moment.

Mr Dunne first received an inquiry about Mr Joyce's citizenship from Labour MP Chris Hipkins in a question on notice last Wednesday, asking whether a child born in Australia to a new Zealand father would automatically have New Zealand citizenship.

Mr Hipkins has not responded to calls from NZN asking what prompted his line of questioning.

The New Zealand Labour Party's president Nigel Haworth said he knew nothing of the inquiries.

"I have absolutely no knowledge of that at all," he told NZN.

"I genuinely know nothing about that."

Under section 44 of the Australian Constitution, anyone who holds dual citizenship is ineligible to sit in parliament.

He has referred himself to the High Court which will test his eligibility later in August.

Mr Joyce is the fifth Australian MP to have their citizenship questioned since last month, following senators Scott Ludlam, Larissa Waters, Matthew Canavan and Malcolm Roberts.