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A photo posted by Mr Joyce on his official Facebook page said his father was from Hampden in North Otago.
Solicitor-General Stephen Donaghue told the full bench - sitting as the Court of Disputed Returns - on Tuesday the deputy prime minister was 10 years old when his father discovered his status while trying to become a Justice of the Peace in 1978.
He argued Mr Joyce did not know of those events until August this year, so was not aware that he was a New Zealand citizen by descent or even of the possibility that he could be.
The Commonwealth opened the three-day hearing into the eligibility of seven federal politicians, including Mr Joyce and fellow Nationals Matthew Canavan and Fiona Nash.
It argues the Nationals trio, crossbench senator Nick Xenophon and former Greens senator Larissa Waters should not be disqualified because they did not voluntarily obtain or retain foreign citizenship.
It submits, however, that One Nation senator Malcolm Roberts and Scott Ludlum - who has now resigned as a Greens senator - should be found to be ineligible.
The constitution does not allow dual citizens to sit in Parliament.
The hearing continues.