The country's electoral commission investigated potential breaches of the Electoral Act made by NZ First before making the referral, to the force's Serious Fraud Office.

At the heart of the matter are links between the party and a foundation it used for fundraising.

On Monday, the electoral commission - which cannot make legal findings - sent the matter to police, suggesting the foundation had been improperly used, with donations not disclosed as required.

Just hours after a poll suggesting his party was on the brink of missing a return to parliament, NZ First leader and deputy prime minister Winston Peters on Monday welcomed the referral.

"I am advised that in all its dealings the Foundation sought outside legal advice and does not believe it has breached the Electoral Act," Peters said.

"This party is built on the support of loyal supporters and donors and believes it has followed the law implicitly."

Ardern, who has tied her political fortunes to the maverick veteran through their coalition, asked voters to delay judging Peters' party until the police concluded their investigation.

"I think everyone deserves the know the outcome of this process but it needs to be done properly," she said.

"We need to give the serious fraud office the time and space to do their job and suspend judgment until they have."

The revelation came after the first poll of 2020 showed Ardern's government is on track to win a slight majority and a second term - but without NZ First.

The headline figures from the Newshub-Reid Research poll have opposition National on 43.3 per cent, Labour on 42.5 per cent and the Green party on 5.6 per cent.

NZ First received just 3.6 per cent of the preferences of those polled - below the five per cent threshold needed to return to parliament.

Translated into parliamentary seats under New Zealand's complex mixed member proportional system, Labour and the Greens would claim 62 seats of the 120-member parliament, enough to govern with a wafer-thin majority.

Quite apart from Australian politics where the left-leaning parties have an acrimonious relationship, in New Zealand Labour and the Greens govern together in relative harmony.

Ardern also maintains a huge lead as preferred prime minister in the poll, heading opposition leader Simon Bridges by 39 per cent to 11.

The election will be held on September 19.

Australian Associated Press