New Zealand Labour has flashed its pearly whites for the election campaign, promising free dental care for under-30s.

Prime Minister Chris Hipkins made the pledge at Labour's campaign launch on Saturday in Auckland, where hundreds of red-shirted members gathered to kickstart their run to the October 14 poll.

The occasion was overshadowed by anti-vaccine protesters, with half a dozen infiltrating the auditorium and heckling Mr Hipkins and former prime minister Helen Clark during their speeches.

Both Labour and National party leaders have had campaign stops ruined already this election season by the far-right demonstrators.

On Saturday, Mr Hipkins ploughed on despite the interruptions to outline the dental pledge.

"New Zealand has some of the highest recorded rates of unmet need for adult dental care. Overwhelmingly because of cost," he said.

The government believes 1.5 million adult Kiwis put off visits last year because they couldn't afford it.

Basic dental care is free in New Zealand for under 18s, with low-income and disabled Kiwis also eligible for government assistance.

Labour's plan extends eligibility to 23-year-olds from 2025 and to 29-year-olds by 2026, meaning 40 per cent of the population will be eligible within three years.

The long lead-in time is because New Zealand currently doesn't have the health workforce to support a shift.

The plan, which will cost $NZ380 million ($A350 million) over four years, will focus first on attracting and training dentists.

"Labour knows such a fundamental change in our public health settings needs to be carefully designed," Health Minister Ayesha Verrall said.

"Choosing a start date of July 1 2025 means we have time to enable the sector to prepare, which is why we're rolling out the policy in stages."

The phased in approach is incongruous with Mr Hipkins including the policy in part of Labour's promised 10-point cost of living plan, given it won't take effect for at least two years.

The pledge also takes some of the wind from the Greens, who are offering universal basic dental care, funded by a wealth tax.

Labour needs to claw back lost support if it is to win a third term.

After winning 50 per cent of the party vote under Jacinda Ardern at the 2020 election, it is now polling around 30 per cent and trails National in a tight race.

Pundits see Labour going negative in its efforts to peg back centre-right opponents National, and Mr Hipkins included plenty of attack lines in his 35-minute speech.

"I see the National-ACT-New Zealand First coalition of cuts lining up to take us backwards, removing hard won gains, and stopping progress on so many urgent things that matter," he said.

Labour also trails both National and right-wingers ACT in fundraising.

Radio NZ reports National has raked in $NZ8.2 million ($A7.5 million) in donations since the start of 2021, with ACT taking in $NZ4.2 million ($A3.9 million).

The Greens are next best, with $NZ1.4 million ($A1.3 million), even ahead of Labour on NZ$1.1 million ($A1 million).

While National has been campaigning all year, releasing dozens of policies, Labour has made previous few campaign pledges until this month.

The dental pledge joins other promises, including taking GST off fruit and vegetables to make them 13 per cent cheaper, new vaping restrictions and four weeks parental leave for partners during childbirth.

On Sunday, the campaign focus will switch sides to National, when leader Chris Luxon launches his party's campaign in south Auckland.