Australian Prime Minister Malcolm Turnbull said today the "business of government goes on" despite a citizenship crisis that ousted his deputy and cost the government its parliamentary majority.

Australia's High Court ruled earlier today 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.

"The decision of the court today is clearly not the outcome we were hoping for but the business of government goes on," Turnbull told reporters in Canberra.

Turnbull confirmed that a by-election would be held in Joyce's seat on December 2.

The court decision has cost the coalition government its one-seat parliamentary majority.

Joyce was one of seven politicians whose eligibility to sit in parliament was thrown into doubt when it was found they were dual citizens, which bars them from being elected to the national parliament under Australia's constitution.