Barnaby Joyce knew his father was born in New Zealand but wasn't aware until recently he had been a citizen and renounced it nearly 40 years ago, the High Court has heard.
Solicitor-General Stephen Donaghue told the full bench on Tuesday the deputy prime minister was 10 years old when James Joyce discovered his status while trying to become a Justice of the Peace in 1978.
He came to Australia as a British subject but acquired New Zealand citizenship when it became law in 1949. He renounced it as soon as he found out.
Mr Joyce didn't know that had happened until this August so was unaware that he was a New Zealand citizen by descent or even of the possibility that he could be, Mr Donaghue argued on behalf of the federal government.
"The evidence is that Mr Joyce never knew that his father was a New Zealand citizen," he told the court.
Mr Donaghue opened the three-day hearing into the eligibility of seven federal politicians whose citizenship has come into question.
The constitution does not allow dual citizens to sit in parliament, in a section aimed at ensuring MPs don't hold split allegiances.
The Commonwealth argues that five of those referred - Mr Joyce, Nationals senators Matt Canavan and Fiona Nash, crossbencher Nick Xenophon and former Greens deputy Larissa Waters - should not be disqualified because they did not voluntarily obtain or retain foreign citizenship.
All except Ms Waters were born in Australia and have given sworn evidence they had no knowledge the law of another country had made them citizens by descent.
Mr Donaghue said the constitution was never intended to target those kinds of people and that the relevant section should not be read literally.
"A person may have the status of being a subject or citizen of a foreign power but nevertheless not be disqualified by section 44(i)," he said.
Those who believed there was a "sufficiently high prospect" of having foreign ties, however, should not be eligible - including former Green Scott Ludlam and One Nation senator Malcolm Roberts, both of whom were born overseas.
While Ms Waters too was born abroad, Mr Donaghue said her failure to renounce her Canadian citizenship was "reasonable" because she thought she had a choice to apply for it.
He was surprised that her lawyers argue it was unreasonable and she should be disqualified.
Her case was similar to that of Senator Canavan, who also thought he was eligible for Italian citizenship through his mother's side but had to apply for it.
"The senator thought about it and decided he didn't want to be an Italian citizen and so he didn't fill out the form."
Mr Donaghue wants the court to interpret the constitution's "operational purpose" and take a narrower view than it did in the last case of its kind, Sykes v Cleary, in 1992.
The hearing continues.