The High Court of Australia found Joyce's election untenable given his New Zealand citizenship.
Australia's High Court ruled earlier on Friday that Deputy Prime Minister Barnaby Joyce and four other lawmakers are ineligible to remain in parliament because they held dual citizenship at the time of the last election. "It's a pretty simple story - we're off to a by-election".
"I was always of the view that it was going to be a tough game", he said.
The judges also pointed to the "difficulties of proving or disproving a person's state of mind" and the "regrettable possibility of a want of candor" if ignorance of dual citizenship was recognised as an excuse.
"I had no reason to believe I was a citizen of any other country but Australia".
The decision deals a blow to Prime Minister Malcolm Turnbull's Liberal-National coalition, which has a one-seat majority in the lower house. He is expected to win.
The saga began for Mr. Joyce in July after media inquiries to his office made him aware he might hold dual citizenship through his father, James Joyce.
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Nationals deputy leader Fiona Nash, One Nation's Malcom Roberts and former Greens senators Larissa Waters and Scott Ludlam have also been disqualified.
Six senators could be disqualified, though the balance of power would not change since senators can be replaced without elections.
The seven lawmakers said they did not know they were dual nationals when they ran for election past year.
In Joyce's case, his father was a New Zealander, which according to a New Zealand government website, means he is "a New Zealand citizen by descent".
As soon as they found out, they took all reasonable steps required to sever their foreign ties, Walker said.
"It's a pretty simple story, we're off to a by-election (and) I put myself forward to the people of New England as a candidate", Joyce told reporters in his constituency, a rural part of eastern Australia, north of Sydney.