New Zealand anti-vaccine protesters pelted police with a "stinging substance" sending three to hospital with injuries Tuesday, as tensions spilled over in an angry weeks-long protest.

Prime Minister Jacinda Ardern decried "absolutely disgraceful" scenes after the suspected acid attack, and a startling incident that saw one protester speed a car toward police lines before coming to a quick halt just centimetres away.

The brief but intense confrontations erupted near New Zealand's parliament early Tuesday, as police moved roadblocks used to contain a protest camp that has clogged downtown Wellington for two weeks.

The Wellington protest began as a movement against vaccine mandates -- inspired by similar protests in the Canadian capital Ottawa.

It has since grown to around 1,500 people and encompasses a range of grievances, with some far-right messaging among the anti-government and anti-media slogans on display.

Assistant Commission Richard Chambers said three officers who were taken to hospital were "recovering well".

He blamed a "certain group within the protest" for the "appalling" attack.

"Police officers are going about their work as best they can to bring peace to the situation... there's a group (of protesters) determined to bring violence and aggression -- we can't tolerate that."

On Monday police reported demonstrators hurled human faeces at them, prompting officers to protect themselves with riot shields during the latest clashes.

Ardern said she was concerned the protest was becoming increasingly violent.

"The attacks on the police have been absolutely disgraceful," she said.

"To anyone down there who thinks they're part of a peaceful protest, that's not what we've seen today -- I would encourage them to leave."

Authorities had been taking a largely hands-off approach to the demonstration, trying to persuade protesters to voluntarily move on.

Police hold riot shields as demonstrators against Covid-19 vaccine mandates and restrictions gather outside of the New Zealand Parliament grounds in Wellington, New Zealand, on February 22, 2022
Police hold riot shields as demonstrators against Covid-19 vaccine mandates and restrictions gather outside of the New Zealand Parliament grounds in Wellington, New Zealand, on February 22, 2022 Dave Lintott / AFP / Dave Lintott

But Chambers said recent events showed "genuine protesters are no longer in control of the behaviour in and around parliament".

The protesters, inspired by Canada's "Freedom Convoy", have jammed roads with around 900 cars, trucks and campervans, then set up camp on the lawns of parliament.

They have erected tents and shelters, and organised portable toilets, food distribution points and childcare facilities.

Wellington residents have complained about being abused by protesters for wearing masks, while schools and businesses close to the camp have closed.

"Wellingtonians have had enough of this," Deputy Prime Minister Grant Robertson said.

"Our streets have been blocked, our people have been harassed, our environment has been trashed."

Robertson said those involved in the protest had crossed the line into illegal activity.

"This is a protest that has gone well beyond what I think most New Zealanders would see as a peaceful protest -- you've made your point, please leave now," he said.

Chambers said some moderate protesters had left because they were concerned about the behaviour of more extreme elements and police still hoped for a negotiated end to the demonstration.

"We continue to work really hard with leaders among the group to de-escalate the situation and ensure everyone is safe," he said.

New Zealand authorities have also been in contact with their counterparts in Ottawa, where police in riot gear dislodged the trucker protest over the weekend after more than three weeks.

Chambers said police, who have received criticism from locals for not doing enough to stop disruption, would have a "highly visible" presence around the protest.

"Police do not wish to interfere with lawful protest, but the behaviours we are seeing are unlawful and will result in enforcement action," he said.