Prime Minister Jacinda Ardern has warned that NZ First should never be underestimated

Prime Minister Jacinda Ardern has warned New Zealand to "never underestimate New Zealand First", as her party grapples with its lowest poll numbers in two years.

Last night, a 1News/Colmar Brunton showed Labour's support at just 39 per cent.

National was at 46 per cent and, along with Act's 2 per cent and assuming its leader David Seymour retains his Epsom seat, the poll shows the two parties could form a government.

The Greens were steady at 7 per cent and NZ First had dropped below the 5 per cent threshold, meaning it would fail to get back into Parliament.

Speaking to 1News this morning, Ardern warned "never underestimate New Zealand First".
If NZ First increased its vote by just 1 per cent, it would be back in Parliament.

Prime Minister Jacinda Ardern took aim at the Colmar Brunton poll and said it has consistently underestimated Labour. Photo / Mark Mitchell

National Leader Simon Bridges said that Ardern's comments this morning indicated that Ardern was "effectively saying she's banking on NZ First at the next election".

"Labour-NZ First tied at the hip. It's becoming increasingly clear; a vote for NZ First is a vote for Labour in Ardern's eyes."

Neither National nor Labour have said if they're prepared to work with NZ First to form a coalition after the next election.

But Bridges has promised to rule a partnership in, or out, early next year.

Traditionally, NZ First's poll numbers ramp up closer to election time.

Meanwhile, Ardern remains optimistic about her party's support, despite the low poll numbers.

"We still have the majority of New Zealanders supporting us as a government – that feels to me like it reflects the feedback I get from New Zealanders around the progress we're making."

She pointed out a Stuff poll last week was quite different to the Colmar one so she keeps a "healthy dose of scepticism" when it comes to polls.

That poll showed Labour at 41 per cent and National at 38 per cent.

In fact, Ardern took aim at the Colmar Brunton poll and said it has consistently underestimated Labour.

She said that has been a constant issue with the poll.

"Even the polls that are good I express healthy scepticism over.

"The fact that we still have the majority of New Zealanders supporting us as a government, I feel positive about."