Roughly a million people didn't show up to vote for Saturday's election, making it one of New Zealand's worst turnouts in the last century.
An estimated 77.04 per cent of enrolled voters took part in the election, slightly higher than the 74.2 per cent turnout in 2011, which was the worst in percentage terms since before women got the right to vote in 1893.
This year's result still ranks as the third-worst turnout in the last 100 years, with the number of non-voters almost tallying to the number of votes that went to National.
The estimated results are based on the 2,405,652 votes received before voting closed, which includes nearly 300,000 special votes that are yet to be counted.
Voter turnout has been on a downward slide since the 1960s, when it consistently reached 90 per cent.
It has declined in each election since the advent of MMP, with the exception of 2002.
Victoria University politics professor Jack Vowles said the slight increase was promising in the context of sliding voter turnout.
"A small increase in turnout is what we would expect," he said.
"There's been a downward trend of turnout for some time, so any increase shows something has changed."
The slight increase could have been caused by the appearance of a close race, he said, but the number was still lower than it should be.
Vowles said a key part of increasing voter turnout was for parties to have meaningful differences in policy.
"The policy differences have to mean something to people, and it looks as though they didn't mean enough to potential voters."
He said it was "almost certain" that the turnout of young people was low.
Advanced voting surged at this election, with more than a third of all votes cast in the 18-day period leading up to the election.
Vowles said there was evidence that advanced voting generally had little effect on turnout.
Compared with other countries, New Zealanders are keen voters, with a better record for turnout than most countries, including the United States, the United Kingdom and Canada.
But Australia, Sweden, Denmark and Turkey consistently vote in higher percentages, as did Scotland in their recent independence referendum.
The number of people enrolled to vote also went down at this election. Despite being legally obliged to enrol, only 91.7 per cent of people had done so, down from 93.4 per cent in 2011.
The official election results will be announced on October 4.