The citizenship scandal that has roiled Australia’s Parliament threatened to claim its biggest casualty on Monday after the deputy prime minister was revealed by the New Zealand government to be a New Zealander, unbeknown to him.

The Australian Constitution prohibits people with dual citizenship from holding seats in the national legislature.

Deputy Prime Minister Barnaby Joyce, 50, the leader of the right-leaning National Party, has served in the House of Representatives since 2013, after serving eight years in the Senate. He is also the minister of agriculture, the source of a well-publicized dispute over dogs brought into Australia in 2015 by the actor Johnny Depp.

He is the fifth Australian politician to resign or come under scrutiny over their citizenship. Last month, both deputy co-leaders of the Australian Greens, Scott Ludlam and Larissa Waters, resigned from the Senate after they discovered they were dual citizens. Matthew Canavan stepped down from his ministerial post, though not his Senate seat, after learning his mother had registered him for Italian citizenship, and Senator Malcolm Roberts has been forced to offer evidence that he had renounced his British citizenship after documents showed he had traveled on a British passport.