The landmark ruling by the court that Mr Joyce was a citizen of New Zealand by descent through his father James Joyce and, therefore, was ineligible to be an MP under section 44 of the constitution, will trigger a bitter political contest and means the Turnbull government has lost its one-seat majority in the lower house.

The byelection will be held in Mr Joyce's former northern NSW seat on December 2.

Before the ruling, Prime Minister Malcolm Turnbull repeatedly said he was "very, very confident" that, based on the advice of Solicitor-General Stephen Donaghue, Mr Joyce would be found to have been validly elected.

The court, sitting as the Court of Disputed Returns, swept the Solicitor-General's advice aside and, in an embarrassing blow, ordered Mr Joyce to face a byelection.

Mr Joyce apologised to voters in his seat after the ruling and said he respected the verdict of the court.

"I was always prepared for this outcome. I don't actually stand here totally surprised. I always expected that this was going to be a tough game," he said.

"It is a pretty simple story. We are off to a byelection. I put myself forward to the people of New England as their candidate.

"I had no reason to believe that, you know, I was a citizen of any other country than Australia. That is the way it is."

Fairfax Media has been told that internal Nationals' polling has Mr Joyce on a primary vote of 57 per cent, while his political foe Mr Windsor – who held the seat until 2013 and who was considering running against Mr Joyce again – is on a low 16 per cent.

Mr Windsor, who stood down from 2013 from New England before attempting to win the seat back in 2016, said he would not be standing for his old seat because "as much as I like election campaigns, my wife doesn't" and that "enough is enough". This will significantly improve Mr Joyce's chances of winning the seat.

However, Pauline Hanson's One Nation, the Shooters, Fishers and Farmers party, Labor and the Greens are all considering running candidates in the seat.

Federal Nationals president Larry Anthony was disappointed by the court's decision but said the party had been preparing for a byelection and a campaign office would open shortly.

"You can never take things for granted and byelections, they are always difficult for serving governments, though the circumstances here are slightly different," he said.

"Tony Windsor didn't put his hand up but that doesn't diminish our resolve as there will be a cast of thousands who will put up their hands. I'm confident Barnaby can win, he has demonstrated results for the people of New England."

The Turnbull government will likely face two more sitting weeks of Parliament in which it holds 74 seats on the floor of the house, while Labor and the crossbench hold a combined 74 seats.

Independent MP Cathy McGowan has already indicated that she will continue to back the Turnbull government on matters of confidence and supply, which, crucially, will mean the Prime Minister can count on 75 votes – including Ms McGowan's – of the 149 MPs remaining in the 150-member lower house.

Mr Turnbull said after the ruling that Mr Joyce remained the best person to represent New England and "while I know Barnaby will be disappointed with the outcome of the court case, it's as though he's been let out of the stalls and he's ready and raring to go. His enthusiasm is absolutely infectious".

Nationals senator Nigel Scullion will fill in for Mr Joyce as acting Nationals leader.

Asked about the Solicitor-General's advice – which proved to be incorrect – Mr Joyce said "you don't try and second guess the High Court. They make their own deliberations. I respect their verdict".

Mr Joyce played down internal polling that has circulated about the fate of the seat, saying he would not rely on it.

"I put myself forward as the candidate and I work hard and stay humble. That is what I will concentrate on". He said he expected a range of candidates, including from One Nation.

Mr Joyce paid tribute to his former deputy, Ms Nash, who has lost her seat in the Senate and is likely to be replaced by Liberal candidate Hollie Hughes.

"She [Ms Nash] has been so stoic during these times and had to put up with so much. And I get the chance of a byelection; Fiona doesn't. I'd like to offer my support to senator Fiona Nash," he said.

Mr Joyce referred himself to the court back in August after it emerged he could be a dual citizen of New Zealand and thus ineligible for Parliament.

Nationals MPs have been increasingly nervous the court would rule against Mr Joyce after three days of hearings earlier in October.

BARNABY JOYCE
Nationals leader, Deputy Prime Minister
What the problem was: New Zealand citizenship
What now: Will contest a byelection in the regional NSW seat of New England
Potential replacement: Decided by byelection

FIONA NASH
Deputy Nationals leader, Minister for Regional Development
What the problem was: UK citizenship
What now: May battle to force her likely replacement to stand down so she can resume her seat
Potential replacement: Hollie Hughes

SCOTT LUDLAM
Former Greens senator and co-deputy leader
What the problem was: New Zealand citizenship
What now: Will seek a career outside politics
Potential replacement: Jordon Steele-John

LARISSA WATERS
Former Greens senator and co-deputy leader
What the problem was: Canadian citizenship
What now: May battle to force her likely replacement to stand down so she can resume her seat
Potential replacement: Andrew Bartlett

MALCOLM ROBERTS
One Nation
What the problem was: UK citizenship
What now: Has announced intention to run for Queensland state seat of Ipswich
Potential replacement: Fraser Anning

MATT CANAVAN
Nationals senator, former minister for resources
What the problem was: Italian citizenship
What now: Returns to his previous cabinet position
Potential replacement: No replacement needed given he was cleared by the court

NICK XENOPHON
Independent senator
What the problem was: UK citizenship
What now: Xenophon was cleared by the court but will still leave the Parliament to contest the 2018 South Australian election
Potential replacement: Tim Storer or Rex Patrick