"By-elections are an opportunity for political parties to present their vision...the surprise to me hasn't been that the Greens are running, it has been that National isn't running.
"I'll be out there talking about all the reasons why I would love to be the candidate for Mt Albert, but demonstrating that, at the same time, Labour and Greens have a relationship that means we will be able to work effectively in Government."
Shearer won Mt Albert in 2014 with a majority of 10,656 votes.
National got 14,360 party votes in the seat in 2014, ahead of Labour on 10,823 and the Green Party at 8005.
In announcing the decision not to stand a candidate, Prime Minister Bill English said the seat was a safe one for Labour and his party wanted to focus on the general election.
That decision comes after National candidate Parmjeet Parmar lost heavily to Labour's Michael Wood in the Mt Roskill byelection in December.
The Green Party did not stand in that byelection, a decision reached after Labour and the Greens signed a Memorandum of Understanding, with the aim of changing the Government.
Ardern is a list MP and her probable victory will mean another Labour List MP will come into Parliament. Next on the list are Maryan Street and Moana Mackey, but Labour leader Andrew Little is understood to be keen to get Raymond Huo in to provide Chinese representation.
Ardern recently moved into Mt Albert with her partner Clarke Gayford but had been intending to stand in Auckland Central for the third time prior to Shearer's resignation.
Her shift to Mt Albert will open up the candidacy for Auckland Central, which Ardern was hoping to win from National's Nikki Kaye on her third attempt.
Meanwhile, nominations to be Labour's candidate in New Lynn also closed tonight.
Massey University senior lecturer Dr Deborah Russell is seeking the nomination for the safe Labour seat, vacated by former party leader David Cunliffe, who is retiring from politics.
