​Wellington: The race to form New Zealand's next government took an unexpected turn on Thursday as a "bombshell" poll showed the opposition Labour party had overtaken the ruling National party, threatening its almost decade-long hold on power.

Prime Minister Bill English, a political veteran and former finance minister, faces a highly contested race against Labour's Jacinda Ardern, whose charisma and rising popularity have prompted domestic media to coin the term "Jacindamania".

National fell three points to 41 per cent, its lowest result since 2005, in a poll commissioned by 1 News Colmar Brunton and published on its website only an hour before English and Ardern faceed off in a debate. 

Support for the newly invigorated Labour party jumped six points to 43 per cent, after the last-minute leadership change boosted its chances. It's the first time the Colmar Brunton poll has put Labour ahead of National in 12 years.

"This is a bombshell," said Bryce Edwards, political analyst at Wellington-based Critical Politics. "It does mean that this campaign has turned on its head. National being in second place is entirely unexpected for the last ten years."

Ardern, 37, took the helm of the Labour Party on August 1, after poor poll results prompted the resignation of Andrew Little, breathing fresh air into an election that until then had been widely considered a slam dunk for National.

Speaking in her home electorate of Mount Albert on Friday, Ardern said Labour's own polls did not have her party ahead of National, although they showed the party tracking up.

"It is heartening for us, we are moving in the right direction, but as I said last night, the Colmar Bruton poll did surprise me. We're not taking anything for granted."

National or Labour would still need to strike a deal with smaller parties to form a government.

The Green Party, with which Labour has an agreement to work, edged up to 5 per cent. The two parties would probably still need the nationalist New Zealand First party, which slipped two points to 8 per cent, to form a government. 

Reuters, Stuff NZ