Jacinda Ardern has hit out at National, calling them "desperate liars", as hostilities ramp up on an increasingly acrimonious campaign trail.

A clearly angry Ardern said an ad campaign by National which attacked Labour over tax was "full of lies and scare mongering".

But National leader Bill English stands by the campaign and says National is asking the same questions as everyone else who wanted to know how they would be affected by Labour's tax plans.

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National leader Bill English says his party is asking the same questions as everyone else about Labour's campaign.

The Labour leader's comments follow a shock Newshub-Reid Research poll suggesting the party's momentum has stalled, amid signs National's unrelenting attack on tax are gaining traction.

READ MORE: 
* Voters punish uncertainty, and Jacinda Ardern's left enough of it for National's attacks to work
* National drops to 39 in new bombshell poll, Labour remains ahead
* National still top with small businesses, but Ardern factor shifts votes towards Labour
* Memo to (tactical) voters: There are three main trains running towards election night

The poll had Labour adrift of National slipping 10 points behind National at 47.3 per cent to 37.8 per cent. National had climbed four points in the poll suggesting the mood for change unleashed by Labour's change of leadership may be running out of steam.

Bill English - leading the charge on attack ads.

But the polls have been hugely volatile and internal party polls appear to show voters chopping and changing their vote to an unusual degree, so the election is still anyone's game.

Labour has been under seige over its tax plans, with National capitalising on uncertainty over its intentions by claiming it plans to introduce a raft of new taxes including a capital gains tax, land tax, inheritance tax, income taxes, water taxes and others.

The party has ruled out many of them including raising income taxes and a capital gains tax on the family home but has failed to get traction on its denials, in part because Ardern has deferred many decisions to a tax working group once Labour is in office.

MAARTEN HOLL/STUFF

Jacinda Ardern - calling out her opponents as desperate liars.

National's new ad campaign is titled "let's tax this" and claims new taxes on a huge number of goods and services under Labour.

Speaking in Nelson, Ardern said she was calling National out on its "lies" and "scaremongering" and the New Zealand public deserved better from National.

"I will run an open and relentlessly positive campaign but they need to start running an honest one," she said.

But English was unapologetic on the campaign trail in Auckland.

"What surprises me is the Labour Party don't seem to think it matters but actually this is about the cash in peoples' pockets, $1000 a year, in or out of someone on the average wage's pocket, other thousands of dollars affecting all sorts of businesses, families, and homes through all the other taxes that they proposed. "

It wasn't scaremongering to ask questions Labour couldn't answer, English said.

National disagreed with Labour's claim, for instance, that it was not increasing income taxes because Labour had made it clear it would scrap next year's tax cuts.

"That's been legislated fpr and supported by NZ First and the Greens so those tax reductions have been locked in by a big majority of the Parliament (for) April 1. If Labour cancels them they're taking $1000 out of the pocket of someone on the average wage."

He also rejected the suggestion that Labour had dealt with questions about an inheritance tax.

"Now they've got to explain what actually happens when a house gets sold that's been inherited. Will it be subject to a comprehensive capital gains tax because family aren't living in it and its not a family home?"

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