Newshub poll: Jacinda Ardern overtakes Andrew Little again as preferred Prime Minister

With Andrew Little's leadership of the Labour Party in doubt, Jacinda Ardern has leapfrogged him in the Preferred Prime Minister stakes - again.

While Mr Little has held fairly steady, up by just 0.1 percent and settling at 7.1 percent, Ms Ardern has leapfrogged him once more, jumping 1.8 percent to sit at 8.7 percent.

It's the second time Ms Ardern has overtaken her leader. When she was first announced as deputy, she came in at 10.5 percent - more than 2 points ahead of her leader.

She then took a dip down to sit almost level with her boss, but has since rebounded back up.

Newshub poll: Jacinda Ardern overtakes Andrew Little again as preferred Prime Minister

But Ms Ardern is standing beside her leader - at least until the election, she says.

"I'm his deputy, I'm ready to work alongside him to take us through to the election," she told Newshub.

Ms Ardern said Mr Little had discussed poll results with her, but she was continuing to support him as leader.

"[There's] no plan B. I've given Andrew my absolute support as deputy and I'll keep doing that," she said.

Mr Little has been struggling in the polls and earlier on Monday, admitted he'd spoken to the party's senior MPs about whether he should resign.

Meanwhile New Zealand First leader Winston Peters has cemented his position as kingmaker in the upcoming election, according to the latest Newshub-Reid Research poll.

Mr Peters now sits at 11.9 percent, up 2.2 percent from the last poll in June - above both Ms Ardern and Mr Little.

He's still trailing a fair distance behind current Prime Minister and National leader Bill English - but Mr English has dropped slightly, down 0.4 percent to 25.8 percent.

The Newshub-Reid Research poll was conducted July 20-28. 1000 people were surveyed, 750 by telephone and 250 by internet panel. It has a margin of error of 3.1 percent.