National leads Labour 46% to 35.8%, according to a preliminary count released by the country's Electoral Commission
. The commission put the populist New Zealand First Party third with 7.5% of the vote and the Green Party fourth with 5.9%.
The results could put Prime Minister Bill English's National Party in a position to remain in power for a fourth term despite a strong challenge from Labour -- a party that had led in some polls preceding Saturday's election.
Without an outright majority, National would have to form a coalition with another party to remain in power. English has said he will discuss the issue with New Zealand First, Radio NZ reported.
Coalitions are nothing new for National, which has run a minority government since 2008.
The results position Winston Peters, leader of New Zealand First, as the kingmaker. His party will decide the outcome of the election by choosing which of the other parties to form a coalition with, said Chris Bramwell, deputy political editor of Radio NZ.
"It's up to New Zealand First, pretty much," said Bramwell.
The Green Party will likely align itself with Labour, Bramwell said.
But the process could take time. Peters told supporters Saturday night that any decision about a coalition wasn't imminent, Radio NZ reported.
"I will not be giving any answers tonight or tomorrow until I've had a full chance to talk to our full board of New Zealand First, our supporters and organizations around the country and our (members of Parliament)," Peters said, according to Radio NZ.
English had seen his party's standing in the polls plummet since announcing the election in February, and Jacinda Ardern's Labour Party surged.
The official results of the election won't be released until October 7
, when special votes -- ballots cast overseas or by those who enrolled and voted at the same time, Bramwell said -- are fully accounted for.
Peters has previously indicated he would make his decision before October 12, according to Radio NZ.
At the start of the year, English -- who replaced former National leader John Key in December
-- was in a strong position and looked certain to continue Key's electoral success.
But a surprise decision by then-Labour leader Andrew Little to stand aside at the start of the campaign made room for Ardern, 37, a three-term member of Parliament.
After she took control of Labour, the party surged in the polls, leading the press to coin the term "Jacindamania."
"Suddenly an alternative Labour-led government (was really) looking viable," Grant Duncan, an expert on New Zealand politics at Massey University, told CNN earlier this month.
"It's possible," Bramwell said, that the Labour Party could gain power in a scenario where the party aligns with the Greens and gains the support of New Zealand First. Even if that happens, Bramwell said, "they may still have to wait for the special votes to be counted" before a final determination could be made.
Polls heading into Saturday's vote put the two main parties neck and neck, with a slight advantage for National in some surveys, according to Radio NZ
Voter engagement was high, with more than 800,000
taking advantage of early voting by Thursday, almost double the number who had done so at an equivalent point in the last election.
The election was largely focused on the economy, with National attempting to highlight its positive record, and housing prices, a key topic of attack for Labour.
New Zealand has the fastest-rising house prices among major developed countries, according to data from The Economist, with a 13% increase in the past year. Labour had promised to build 100,000 new affordable homes if elected.