New Zealand opposition looks

New Zealand opposition leader Jacinda Ardern made a U-turn on tax policy Thursday in a bid to shore up support amid fears her charmed election campaign run is over.

Ardern had previously refused to release a tax plan ahead of the September 23 election, saying she would consult with a committee of experts after the poll.

But the policy vacuum allowed Prime Minister Bill English to attack Ardern's financial credibility, halting the momentum she developed after taking over the centre-left Labour Party last month.

English's conservative National Party ran an effective scare campaign warning voters Labour could introduce taxes on assets like the family home and inheritances, despite denials from Ardern.

In an effort to reignite the "Jacinda-mania" of recent weeks, Ardern said Thursday there would be no changes to the tax system until 2021, taking the issue off the agenda for the current campaign.

"I heard from voters that they wanted that certainty," Ardern told reporters.

The move came as opinion polls provided mixed messages for the charismatic 37-year-old, who took over the Labour leadership with the party's support at 20-year lows of 23 percent.

But while the party is still in a strong position, the polls indicate Labour's huge gains under Ardern have halted, or even declined.

The latest 1News-Colmar Brunton poll, released Thursday, put Labour at 44 percent -- up one percentage point on the previous week but unable to extend its four-point lead over National on 40 percent.

The survey of 1,000 voters also showed Ardern's rating as preferred prime minister slipped one point to 34 percent, while English rose one point to 32.

Even more sobering for Ardern was a Reid-Newshub poll released Tuesday that had National up 4.0 percentage points at 47.3 percent and Labour down 1.6 at 37.8 percent.

Radio New Zealand, which collates various polls, said the average from the three most recent surveys available put National on 41.3 percent, just ahead of Labour on 40.5.

Ardern said it was going to be a close contest.

"Given the volatility of the polls, I'm not willing to accept any of the numbers I'm seeing at the moment," she said.