Mt Roskill has overwhelmingly voted for Labour's Michael Wood to become its next MP, a result Labour leader Andrew Little said was a "bloody nose" for the Government.
Wood was the favourite but his team was thrilled with the size of his victory - capturing 66 per cent of the vote (11,170) to National list MP Parmjeet Parmar's 28 per cent (4652).
Turnout was just under 39 per cent. Special declaration votes still to be counted.
In 2014, long-serving MP Phil Goff secured 57 per cent of the vote, to Parmar's 32 per cent.
Prime Minister John Key said this week Little's leadership couldn't survive if Parmar won, and Little singled out Key in his comments after Wood's victory.
"He came here flouncing around and he seemed to forget that the purpose of an election campaign is to win votes, not selfies."
Wood said he didn't expect such a big win.
"To end up 6500 [votes] ahead is an enormous swing. I want to thank all the people of Mt Roskill who have put their trust and confidence in me and Labour."
Wood will now take the reins from Goff, who retired from Parliament to become Auckland Mayor.
Wood told the crowd, which included deputy leader Annette King and MPs including Phil Twyford and Grant Robertson, that Goff had been a mentor but questioned his taste, given he had left the event early to attend a Coldplay concert.
He thanked his wife Julie Fairey first, and then campaign manager Shayne Misselbrook. Just 27 years old, Misselbrook will start a new job at Meredith Connell on Monday.
Businessman Roshan Nauhria, founder and candidate for the NZ People's Party, came third with 709 votes.
Apart from a three-year break between 1990 and 1993, Goff has been the Mt Roskill MP since 1981, and had an 8000 majority in 2014.
In 2014, National got 14,275 party votes - 2000 more than Labour - but a quarter of National voters opted for Goff.
Goff's departure gave National the chance of an upset win, but a Labour campaign that focussed on turning out its supporters has seen him easily fend off the challenge from Parmar.
Speaking to media at her event at Frolic cafe in Royal Oak, Parmar said she was disappointed in the result but proud of her campaign: "History wasn't on my side and history was proven right."
She said she would be putting her name forward to contest the seat again.
Steven Joyce took the opportunity to have a dig at Little.
"I'd be keen for him to take as much as possible out of tonight but I don't think you can read too much into it. It's a safe Labour seat. The Greens, New Zealand First didn't stand and it's never been done for a Government to win an opposition seat in a byelection."
Little pre-empted that response in his speech to the Labour crowd. Despite any "spin", he said National thought they could win, evidenced by the Prime Minister's seven trips to Roskill during the campaign.
"They had one thing going for them, and it was the Prime Minister. And they have lost. The result tonight is absolutely outstanding.
"For a seat where National won the party vote in the last election, this is a real wake-up call to them."
Who is Michael Wood?
Wood has been on the Puketapapa local board for six years and has previously run Goff's campaigns. He lives in Roskill South with his wife Julie Fairey, also a local board member, and three sons.
The 36-year-old is a former president of Young Labour, who worked for the financial services union, Finsec, and the Amalgamated Workers Union in the early 2000s.
He has previously stood for Labour in Epsom and at the Botany byelection.