Labour leader Andrew Little admits winning the Northland electorate will be an uphill battle - but believes the party could benefit from Northlanders' sense that National has taken their votes for granted.

Mr Little was in Kawakawa on Sunday for the launch of Labour candidate Willow-Jean Prime's campaign.

The by-election, triggered by National MP Mike Sabin's resignation, is on March 28 and will cost taxpayers up to $1 million.

Mr Little used his speech to praise Mrs Prime's passion and commitment to Northland, and quipped that National believed it could "pin a blue ribbon on a donkey" and still win the seat.

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"We know it's going to be an uphill battle, but I detect a sentiment up here that people are being taken for granted and that National has mucked them around in terms of representation," he said.

"You never know with by-elections. They can be seen as a way of sending the Government of the day a message."

If Labour did not win he hoped to at least greatly erode National's majority. The Greens' decision last week not to contest the by-election could benefit Labour but he was not taking their votes for granted.

NZ First's intentions were still not clear but if Winston Peters did stand, National stood to lose the most votes.

Mr Little said he had not had any discussions with Mr Peters about the by-election.

Addressing supporters at the Kawakawa RSA, Mr Little said Mrs Prime was smart, committed and driven.

"Just a few days after giving birth she was back out at the St John Market here in Kawakawa canvassing for votes and signing up new members ... I remember being a new parent and the thought of doing that and campaigning at the same time is awe-inspiring."

She was making the sacrifice because Northland was being neglected and needed a change, he said. The region's unemployment was amongst the highest in the country and $35 million had been cut from its roading budget, because National believed it could "pin a blue ribbon on a donkey and get it a seat in the House".

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As long as that was the case, the Government had no reason to create jobs or fix roads, he said.

Meanwhile, National is in the process of choosing a replacement candidate from a shortlist of Mita Harris, Matt King, Grant McCallum, Mark Osborne and Karen Rolleston. Party delegates will make their choice on Saturday.

Mana has named Rueben Taipari Porter as its candidate and Act has chosen Robin Grieve.