Former prime minister Helen Clark has called for a thorough review into New Zealand's electoral, campaign funding and voting laws following the extraordinary election result.

Ms Clark has raised concerns over "disenfranchising" enrolment changes, a distortion of the electoral system and a lack of oversight over campaign funding.

"New Zealand could well be deemed by observers to have had a flawed election," she told AAP.

The too-close-to-call election will be decided on November 27, when roughly 400,000 extra votes are added into the count.

Those ballots, from out-of-electorate voters, late enrolments and other non-typical voters, typically favour the left and shift the dial by a few seats.

Tens of thousands of ballots from Australian-based Kiwis are among those to be added in.

These votes will be critical, as preliminary results show 67 seats for the conservative coalition, and 64 for opposition parties and new teal force Opportunity.

However, the number of special votes appear to be considerably down on previous elections, which is in part due to law tweaks.

Chris Luxon's right-wing coalition brought forward the deadline to enrol or make changes to enrolment, arguing by setting an election date in January, Kiwis had plenty of time to update their details.

In a tight contest where the final margin may be one seat either way, that could be prove the difference.

Ms Clark, who led New Zealand from 1999-2008, said "the early closure of enrolment disenfranchised many".

The Labour doyenne is also concerned about distortion of the mixed-member proportional (MMP) electoral system through the overhang result.

New Zealand's usual 120-seat parliament will grow by up to 11 MPs as National's electorate vote held up while its nationwide party vote crashed.

On the basis of its party vote, National would normally be entitled to 35 seats, but instead it is on track to return 41 MPs, materially changing whether the left or right will govern.

"This outcome is not what MMP was designed to deliver - which was a proportional result," Ms Clark said.

The left will also benefit from an overhang, with the Maori Party winning 0.6 per cent of the party vote but winning at least four, perhaps five, of the Maori electorates.

Right-wing parties ACT and NZ First as well as third parties explicitly campaigned for the overhang, urging National supporters to "split" their vote between coalition parties to grow the overall right bloc.

Integrity advocates have raised concerns over funding behind these campaigns, which included backing from the billionaire Mowbray family, one of New Zealand's richest.

Ms Clark said Kiwis deserved more transparency after the electoral system was "gamed".

"There is no transparency around who funded it," she said.

"As well - the electoral funding laws allow gross inequity in funding - money politics at work.

"This raises many issues which should now be the subject of a full review."