UPDATED 4.17PM Labour has launched its campaign for the Mount Albert by-election after officially confirming list MP Jacinda Ardern as their candidate.

Labour leader Andrew Little kicked off the campaign in the electorate on Sunday ahead of what is expected to be a battle between Ms Ardern and Green Party candidate Julie Anne Genter.

The by-election on February 25 in the safe Labour seat is to replace David Shearer, who resigned from parliament last year to lead United Nations peace keeping missions in South Sudan.

Mr Shearer won the seat in 2014 with more than 20,900 votes over National's 10,300 and the Green Party's little over 3000.

As in any by-election, people have only one vote - for their electorate MP. There is no party vote.

"Mt Albert is a vital electorate. It's had outstanding high quality Labour MPs representing it for many, many years and many decades and we're about to make sure that that tradition continues," Mr Little said launching the campaign.

Ardern listed housing and transport as the major issues in her area. She said Mt Albert has a high proportion of younger people, younger families and first home buyers who have either been locked out of the housing market or are paying high prices for rent, so housing and transport are definitely big issues for them. 

Prime Minister Bill English has said Ms Ardern would be "difficult to beat" after revealing National would not run a candidate against her in the by- election.

The party will instead look ahead to contesting the seat in the general election later in the year.

But Ardern said she'll be campaigning as hard as if National was running. 

She's surprised at National's decision saying regardless of which party held the seat in the past, by-elections are an opportunity for parties to share their vision and said it strikes her as odd that National hasn't taken that opportunity,