With 90 percent of the vote in as of 9pm Saturday, National party candidate Andrew Bayly has 13573 votes, while New Zealand First candidate Casey Costello, the only other major party candidate, has 2649, a margin of almost 11,000 votes.
Costello has told RNZ tonight 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."
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.
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.
If Bayly wins the seat, Parliament will have 123 seats - the biggest ever.
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.
