The Northcote by-election will be held on June 9, a week before Prime Minister Jacinda Ardern's baby is due.

Ardern announced the by-election date on Monday.

"There's a reason it's not on the 17th of June, which is my due date," she told reporters.

The by-election is being held because Jonathan Coleman has resigned from parliament, effective on April 15.

Northcote has been a safe National seat in recent elections but Ms Ardern says Labour is in no way resigned to it staying that way.

"Absolutely not," she said.

"We will be undertaking our candidate selection very soon, at the end of this week, and we will be putting up a very strong fight in that seat."

The deadline for candidate nominations to be received is May 15.

The electorate on Auckland's North Shore includes the suburbs of Northcote, Birkenhead, Beach Haven, Hillcrest and parts of Glenfield.

The Electoral Commission expects there will be about 25 voting places for advance and election day voting.

- NZ Newswire