Andrew Bayly Photo: RNZ / Samuel Rillstone
National MP Andrew Bayly has won Port Waikato's by-election, with NZ First's candidate - the only major party to contest the seat - conceding.
With 100 percent of the vote counted, Bayly has won by a margin of 11,245 votes.
New Zealand First candidate Casey Costello, the only other major party candidate, had 2778 votes.
Costello told RNZ on Saturday night that she concedes the race.
"We knew it was a National stronghold but we weren't going to walk away.
"We just think it is important for democracy that voters have some choice so we were committed to making sure that Port Waikato had a choice.
"It's been a great experience. The extra campaign, a lot more connections, a lot more people to talk to, a lot more door-knocking."
The campaign has been great, she said.
"It is my home town and I really wanted to make sure that I got to connect with as many people as possible."
Bayly's win brings Nancy Lu into Parliament on the National Party list.
Lu will be the 123rd MP, the most New Zealand has ever had in one Parliament.
Labour, the Greens and the ACT Party were not standing for the seat.
A variety of small minor parties were also standing.
Voting closed at 7pm.
More than 12,000 people had turned out to vote with two hours to go before voting stations shut.
There have been 37 voting places open for the by-election.
The Electoral Commission said the stations have been operating with no issues to report.
The by-election was sparked by the death of ACT candidate Neil Christensen just before the general election on 14 October.
The seat has long been a National Party stronghold. At number 15 on the party list, Bayly is an MP no matter what the results are today, as is Costello, who is number 3 on NZ First's list.
Casey Costello, right, and Winston Peters at the launch of New Zealand First’s campaign for the Port Waikato by-election. Photo: Katie Scotcher / RNZ
Former Labour MP Sue Moroney said adding a few more MPs won't change too much.
"Doesn't necessarily change the way in which Parliament operates. It simply changes the numbers available for voting and of course increases the cost to the taxpayer slightly.
"So that's something that should be brought in mind when the new government thinks about how they address this for the future."
There had been concerns over voter turnout, with public confusion over another election being held just six weeks after the general election.
The official results, including special and overseas votes, will be released on 6 December.