Voters in Auckland's Mt Roskill electorate are deciding on Saturday who'll replace Labour's Phil Goff as their MP.
The seat was vacated when Mr Goff won the Auckland mayoral election, and resigned from Parliament.
He had held the seat since 1981, with a three-year break between 1990 and 1993.
Mr Goff retained the seat in the 2014 general election with a majority of more than 8000 votes.
National and Labour are the only parliamentary parties contesting the by-election.
There are 47,266 enrolled voters in the Auckland suburban electorate, and 22 voting places which are open from 9am to 7pm.
Around 3500 advance votes have been cast.
The Electoral Commission expects to have half the votes counted by 9pm and the final result by 10pm.
The candidates are: