Prime Minister Malcolm Turnbull says he's sure his deputy Barnaby Joyce will survive a High Court investigation into his dual citizenship, but what happens if he doesn't?

Key points: Mr Joyce could renounce his citizenship then quickly recontest a by-election

Mr Joyce could renounce his citizenship then quickly recontest a by-election Tony Windsor was considering a challenge in the event of a by-election

Tony Windsor was considering a challenge in the event of a by-election Fiona Nash would likely assume Mr Joyce's cabinet responsibilities

That's a question some are contemplating at Parliament House because a by-election could threaten the Government's majority of one in the Lower House, potentially forcing a hung parliament.

On Monday, New Zealand's Prime Minister confirmed Mr Joyce was a citizen of the country because his father was born there.

Several Nationals MPs have told the ABC there's no discussion about replacing Mr Joyce and that no-one is running the numbers, but the effects of a by-election are clear for the Government.

What if the court rules against Joyce?

ABC election analyst Antony Green says if that happened, Mr Joyce could renounce his citizenship and then quickly recontest the seat at a by-election.

"You'd imagine the Government would call that as quickly as possible after the court judgement, which would make it harder for anybody to run a campaign," he said.

Former independent Tony Windsor — a former National who fought a bitter campaign against Mr Joyce at the 2016 election — hasn't ruled out another shot at the seat.

"I've always travelled through life not ruling things out, but I'll be watching this," he told the ABC.

"The issues that I stood on last time against Mr Joyce have gone from bad to worse."

You'd need money to beat the Deputy PM

But a short time frame would make it hard for Mr Windsor to raise the funds and volunteers needed to mount a successful campaign against a deputy prime minister.

The high stakes of a by-election could see major parties splurge money on campaigns to try to wrestle control of the Lower House.

Malcolm Turnbull had told Question Time he was confident Barnaby Joyce would not be disqualified ( AAP: Lukas Coch )

Mr Windsor was able to raise nearly half a million dollars for the 2016 federal election — with a $200,000 donation from entrepreneur Graham Wood — but he still fell short at the polls.

The seat of New England is considered very safe for the Nationals and there would need to be a significant swing against Mr Joyce for him to lose his seat.

As Graham Morris, a former chief of staff of John Howard, points out, any by-election would almost certainly return Mr Joyce given his popularity and senior rank in the Government.

"The people of New England quite like having a deputy prime minister and he would be back, so nothing would change," he told Sky News.

Who'd take over in the interim?

Nationals sources say Mr Joyce's ability to move Government agencies like the Australian Pesticides and Veterinary Medicines Authority to Armidale — despite a backlash from public servants and Labor — shows he is able to deliver for his electorate.

If Mr Joyce was forced to recontest his seat, his deputy, Senator Fiona Nash, would likely assume his Cabinet responsibilities in an acting capacity.

The leader of the Nationals is historically based in the Lower House, but some in the party say that won't stop her acting in the role.

There would also be a question of who assumes Senator Matt Canavan's former duties as resources minister, which fell to Mr Joyce when he stepped aside due to his own citizenship troubles.