New Labour MP David Shearer coasted to an enormous win in the Mt Albert by-election last night, bolstered by congratulations from his predecessor, former prime minister Helen Clark in the Congo on her first development mission for the United Nations.

Interim results showed Shearer had won a 9187 majority - nearly as big as Clark's 10,351 majority in last year's general election.

"I'm feeling, I guess, sort of nervous and relieved and pretty happy, all at the same time," Shearer said.

Down the road at Melissa Lee's National Party campaign headquarters at Tadka restaurant, supporters sang the national anthem and watched the rugby. The candidate shut herself in a back room and refused to come out for half an hour.

When she emerged, she didn't ask the result: she knew she would lose.

"The reflection is on the Labour Party," she said. "Anything less than a landslide result reflects poorly on them." Shortly afterwards, she learned the extent of that landslide.

Despite a little National Party hype early in the Mt Albert by-election campaign, it had quickly become apparent that the result was a foregone conclusion.

But it took the Waterview connection motorway, the Supercity and a string of gaffes from Lee for Shearer to almost replicate Clark's enormous majority. And Clark had served 28 years as MP, nine as Prime Minister, and was up against an almost unknown candidate.

Clark texted her congratulations to Shearer's campaign team for making Mt Albert "a fortress for Labour".

Lee, in a turquoise embroidered satin top, was close to tears as she took the blame for the magnitude of the loss. "The mistakes I have actually made in the campaign have surely had a clear impact on the outcome, and I take responsibility for that."

Up the road in Sandringham, 200 celebrating Labour supporters ate marshmallows, while Shearer and leader Phil Goff pored over a laptop.

Goff: "You're four to one against National in that booth."

Shearer: "I like that result."

Goff said the National Government had not been listening to the people of Mt Albert. "This was a by-election that John Key initially thought he would win," Goff said. "That's why he parachuted Melissa Lee in as a candidate."

Then he quietly asked: "Who's winning the rugby?"