Mt Albert's new MP Jacinda Ardern said the hard work is just beginning, now that she's won the by-election.

LISTEN ABOVE: Andrew Dickens talks to Mt Albert MP Jacinda Ardern and runner up Julie Anne Genter. 

She beat the Green candidate Julie Anne Genter by more than eight and half thousand in last night's preliminary count.

National didn't put up a candidate and third place went to Geoff Simmons, who stood for the Opportunities Party.

Ardern said it's nice to win, but she'll have to do it all over again in September.

"Certainly not every election the government chooses not to stand, so it's hard to read too much into it, but for us it was really about laying a really good foundation for the general election."

Meanwhile, although he's pleased with the by-election result, Labour leader Andrew Little is in no hurry to promote Ardern.

Mr Little said he's aware of speculation Ms Ardern might be promoted to deputy leader, as a reward.

He said the job isn't vacant anyway, so there's no promotion on the table just now.

Instead, Mr Little hopes the size of Ardern's winning margin win will boost Labour's confidence. 

He said what it did demonstrate is the Labour vote is strong, and it's set the party up well for September's general election.