Labour's post-election debrief will include a close look at Auckland - if voters there decided the election alone then National would have the numbers to form a government by itself.

National's preliminary party vote percentage in Auckland is 48.7 per cent - compared to 44.9 per for the rest of the country.

The party vote for National in Auckland in 2014 was roughly the same.

In Saturday's election Labour still recorded big increases in its share of party vote across most Auckland electorates.

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However, the only electorates where National's share of the party vote increased significantly from the last election are in Auckland - including in Labour strongholds such as Manukau East and Kelston.

Labour leader Jacinda Ardern today said the party would look over where it strengthened and where it could have done better.

"We want to make sure we continue to present a vision that demonstrates particularly to Aucklanders how we can resolve some of their two biggest issues which was housing and transport."

The resurgence of Labour under Ardern was underlined by big increases in party vote when compared to the 2014 election.

One of the biggest swings away from National was in Auckland Central. National still won the party vote on 40.4 per cent but Labour was not far behind on 37 per cent. In 2014, National got 45 per cent of the party vote and Labour 22 per cent.

National's share of the party vote also dropped in Mt Albert, Epsom, Tamaki, Northcote, Helensville, and Rodney.

However, it was more than two points higher in 10 Auckland electorates - Mt Roskill, Papakura, Kelston, Te Atatu, Manurewa, Mangere, Pakuranga, Botany, Manukau East and New Lynn.

Labour's share of the vote in all those electorates increased by more - but came largely at the expense of the Greens and New Zealand First.

Botany, Manukau East, Mt Roskill, Pakuranga, New Lynn, Mangere and Manurewa are all in the top 10 electorates with the most overseas-born voters.

No other electorates in the country saw National's share of the vote increase by more than two points.

Te Atatu MP and Labour's campaign chair Phil Twyford said Labour would look further at the reasons for the support changes across Auckland. But he believed it was mainly a result of the squeeze on the smaller parties such as NZ First and the Greens as their supporters got behind Labour or National.

"In the inner city, liberal seats like Auckland Central, that was tough on the Greens because a lot who voted Green in the last few elections came home to Labour."

He said National had picked up support in traditional Labour South Auckland seats, but the apparent "swing" was exaggerated because it was from a very low base. It was similar for Labour in the National stronghold of East Coast Bays, where what looked like a massive swing for Labour had been it moving from 12 to 22 per cent.

Housing and transport were the main policies National and Labour campaigned on in Auckland. Twyford said those were the litmus test issues for Auckland. Labour leader Jacinda Ardern's first policy announcement was an Auckland-wide regional fuel tax to pay for light rail to the airport.

National also had rail but campaigned largely on roading - it opened the Waterview Tunnel in the lead-up to the campaign and that served to highlight the potential success of big roading projects and was referred to often by English during the campaign.

The increase in support for National nationwide may also have reflected underlying concern among home owners about house prices falling, interest rates rising and a potential capital gains tax under a Labour Government.

National could also have picked up support from a backlash against Labour's harder line on immigration. In housing, Labour had put up a KiwiBuild proposal to build 100,000 new houses for first home buyers - half of which were to be for Auckland.

That was up against National's offering of more generous subsidies for first home buyers on middle and low incomes under Home Start grants. National believed that changes to planning rules and Auckland Council's Unitary Plan were already bearing dividends in opening up land and boosting supply.

Auckland Central MP Nikki Kaye said there were two reasons for the swing to Labour and resulting drop for National and the Greens in Auckland Central.

The first was it included the CBD where the left tended to fare well because of younger voters and students. Labour had also done well in Christchurch Central and Wellington Central.

The other reason was Labour leader Jacinda Ardern, who had stood in Auckland Central in 2011 and 2014 so already had a very high profile. Kaye's winning majority over Labour's Helen White on election night was 1519 votes - stronger than the 600-vote majority she got against Ardern in 2014.

"I have had Jacinda in my electorate for seven years, so I'm pretty delighted about the fact I've been able to not only increase my majority against the swing at the party vote level."

She believed a number of Labour and Green Party voters had voted for her as the electorate MP over Labour's Helen White because of her track record in the seat.

The 385,000 special votes are yet to be counted for this election. Labour and the Greens are hopeful of picking up extra seats when they are confirmed. That could help in any coalition negotiations with New Zealand First, which holds the balance of power.