The Labour Party is proclaiming its Mt Roskill by-election win as a major upset against National, even though the party of government never expected to win.

Labour candidate Michael Wood won a decisive landslide victory yesterday in one of the party's stronghold seats of Mt Roskill, Auckland, which was vacated by Phil Goff when we won his mayoralty campaign.

Wood secured 66 percent of the vote against National list MP Parmjeet Parmar's 28 per cent.

NZ People's Party candidate Roshan Nauhria ran third with 709 votes.

Labour leader Andrew Little said that even with the backing of Prime Minister John Key, who campaigned with Parmar on Thursday, National couldn't secure a win.

"They threw everything at this Mount Roskill campaign, they thought they were going to win," Little told the victory party last night.

"They had one thing going for them, and it was the Prime Minister. And they lost."

However, while doing the rounds on Thursday, Key said there was "relatively little" at stake for National in the by-election, and "the expectations would be that we would lose."

National's campaign chair Steven Joyce said as much last night, insisting it was important they fielded a candidate anyway.

"We always put up a candidate," Joyce said. "Parmjeet's a quality candidate, she does a good job for the Mount Roskill electorate as the list MP and the team ran a good campaign but it was always going to be a hard one to win."

"It's hard to get people to turn out for a by-election, so I think that the result is a forgone conclusion."

"People can read into as much as they like but I wouldn't read into it too much because of the nature of the contest."

Parmar contested the seat at the 2014 General Election, winning only 31 percent of the vote. Despite losing for a second time, she has said that she will stand in the electorate again. Joyce confirmed that she will continue to be an option.

Michael 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 by-election.

Just 31 percent of voters had their say, well down from the 73 percent of Mt Roskill residents who voted in the general election.

There are 47,266 enrolled voters in Mt Roskill but only 16,857 went to the ballot boxes compared with 33,392 in the last general election.