As it happened
Prime Minister John Key has congratulated Labour's Michael Wood on winning the Mt Roskill by-election.
But he says the result is not all bad for National supporters, with National's Parmjeet Parmar - who lost the by-election in a landslide - will still represent the voters of Mt Roskill well in her capacity as a list MP.
"Dr Parmar is an outstanding MP who is already a strong advocate for Mt Roskill as a List MP based there," Key said.
"She fought a hard campaign focused on the issues that matter but by-elections are always tough, especially when the seat has been held by your opponent's party for some time.
"Mr Wood also ran a good campaign and I congratulate him on his win.
"Dr Parmar will continue to serve her community as a List MP based in Mt Roskill and the National-led Government will continue to work hard for New Zealanders and their families to help them get ahead."
9.35pm: Speaking to media, Parmar said she was disappointed 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.
"But it's the party's decision so I won't preempt anything."
She said she'd called Michael Wood to offer her congratulations.
Steven Joyce took the opportunity to have a dig at Labour Party leader Andrew 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 by-election."
He said Parmar had proven herself a very good candidate despite the odds being stacked against her.
"We know that next year's an entirely different context and we'll all have to work very hard next year. It doesn't say anything positive or minus about next year it's just literally a by-election which we knew was probably going to go this way."
He said when Parmar went back to Wellington there would be a whole new level of respect for her.
9.08pm: Labour leader Andrew Little tells crowd that National threw everything at the campaign and lost badly.
"For a seat where National won the party vote in the last election, this is a real wake up call to them."
He said next year would be tough and dirt would be thrown at Labour, and took a swipe at some in the media "who just don't get it".
Little told a fired up crowd that National would try and spin the result but there was not getting away from its significance - or the effort they put in to try and wrest it from Labour.
"They thought they were going to win. We had the trash talk...but the Prime Minister doesn't come here for seven days of the campaign, and three days this week without thinking he is going to win.
"They had one thing going for them, and it was the Prime Minister. And they have lost...the result tonight is absolutely outstanding."
At Parmjeet Parmar's election night party, the faithful had all but conceded defeat by 8.30pm.
About 100 supporters nonetheless toasted the party's campaign at Frolic Cafe opposite Cornwall Park, where leading National Party figures Steven Joyce and Paula Bennett lent their support.
"I think Parmjeet has just done an excellent job but at the end of the day, yeah, it's landed about where we expected it to," Bennett told the Herald.
"By-elections aren't known for being too unpredictable - they're known for returning the party that's already there and that's what they've done this time. Really low voter turnout, from what I can see. I think people were more interested in other stuff that's been going on."
Michael Wood is giving his victory speech, with special praise for Andrew Little, wife Julie, campaign manager Shayne Misselbrook, and Phil Goff.
A large crowd of supporters are in euphoric mood, whistling and clapping every other word.
8.54pm: A disappointed Parmjeet Parmar has taken defeat in her stride, saying National always faced an uphill battle winning what has long been considered a Labour Party stronghold.
"It's disappointing but we knew heading into this by-election that it's always hard for an adjustment to win a seat from the opposition. And we are talking about a safe Labour seat here."
8.50pm: Labour's Michael Wood has secured the Mt Roskill by-election with a landslide 6518 win over National's Parmjeet Parmar.
The Electoral Commission has confirmed that 100 per cent of cast votes have been counted, with Wood polling 11,170 votes. Parmar was a very distant second, returning 4652 votes.
Roshan Nauhira, of the NZ People's Party was the third-highest polling candidate, with 709 votes.
Andrew Leitch (Democrats for Social Credit) received 125 votes, Brandon Stronge (The Cannabis Party) received 79 votes, Richard Goode (NAP) received 40 votes and Tua Schuster (Independent) received 32 votes.
8.49pm: Parmjeet Parmar has conceded defeat in the Mt Roskill by-election at her election night party in Epsom.
8.41pm: With just 10 per cent of the vote in the Mt Roskill by-election to be voted, Labour's Michael Wood holds a 5320 vote lead over National's Parmjeet Parmar.
Wood has polled 8957 votes, to Parmar's 3637.
With 82.1 per cent of the vote counted, Labour candidate Michael Wood has extended his lead to a huge 5017 margin.
He now has 8322 votes, and well on his way to a landslide win over National's Parmjeet Parmat well back with just 3305 votes.
8.26pm: The crowd is building at Wood's function and it now feels like a party. The candidate himself is yet to arrive, but MPs in attendance include Jenny Salesa and David Cunliffe, and Matt McCarten, who runs the party's Auckland office.
8.23pm: Labour's general secretary Andrew Kirton has called it for Wood, tweeting, "That's it - we've got it".
With 60 per cent of the vote in, Labour's Michael Wood has extended his lead to 3519 votes.
Wood has polled 6230 votes, with National's Parmjeet Parmar a distant second on 2711 votes.
Roshan Nauhira, of the NZ People's Party is the third-highest polling candidate, with 376 votes.
Andrew Leitch (Democrats for Social Credit) has 376 votes, Brandon Stronge (The Cannabis Party) has 42 votes, and Tua Schuster (Independent) and Richard Goode (NAP) both have received 17 votes each.
8.15pm: Fifty-three percent of votes have been counted in the Mt Roskill by-election and the count is making good reading for Labour supporters and its candidate Michael Wood.
Wood holds a commanding 3191 vote lead over National's Parmjeet Parmar.
In total, 8184 votes have so far been counted, with Wood gaining 5452 votes.
8.12pm: Jenny Salesa, MP for Manukau East, said the early results were very encouraging.
"What we don't know of course if whether those areas are Labour areas. I'm pretty confident but that's more because I was out door-knocking quite a lot, and a lot of the folks I spoke with said they would vote for him."
A convincing victory for Wood would deliver a boost for Labour going into next year's general election, Salesa said.
"There is no doubt that it would be a boost for Labour."
Labour's Michael Wood has now extended his lead to almost 3000 votes with 46.4 per cent of the votes counted.
His vote count stands at 4940, with National's Parmjeet Parmar receiving 2002 votes.
8.05pm: With 42.9 per cent of the vote counted, Labour's Michael Wood looks set for a comfortable win in the Mt Roskill by-election.
Wood now has a 2696 vote lead over Nationa's Parmjeet Parmar.
Wood has received 4558 of the counted votes, compared to Parmar's 1862.
8pm: Michael Wood's lead has extended to more than 2000 votes with 35.7 per cent of by-election votes counted.
Wood has now received 3774 votes, with National's Parmjeet Parmar trailing well behind on 1683.
The margin stands at 2091.
The People Party's Roshan Nauhria is in third spot with just 211 votes.
7.52pm: Auckland Mayor Phil Goff - whose resignation from Parliament is the reason for today's by-election - is at Wood's function.
"I'm very confident, I think that Michael Wood is a very good candidate - he has been a top polling candidate on the local board, he knows the area, he lives in the area, and he is active in the area - I think he will get a good result tonight."
Goff has kept a strong grip on the electorate despite Labour losing the party vote in 2014 to National, and boundary changes that have included more National-leaning suburbs.
He said in a by-election the result would always tighten, but was confident that given time Wood would appeal to both National and Labour supporters.
32 per cent of the by-election votes had been counted.
Michael Wood's lead had extended to 1836 votes, with 3512 votes compared to National's Parmjeet Parmar's 1676 votes.
7.38pm: Twenty-five per cent of the votes had been counted by 7.36pm, with Labour's Michael Wood holding a 1806 vote lead over National's Parmjeet Parmar.
Wood had 3294 votes, to Parmar's 1488.
The third highest polling contender was the People Party's Roshan Nauhria, who had received just 191 votes.
7.25pm: Labour's Michael Wood has surged into an early lead with 21 per cent of the votes counted.
As of 7.20pm, 3167 of the 4915 votes counted were for Wood. National's Parmjeet Parmar had received 1465 votes; giving Wood a 1702 margin.
Voting has closed in the Mt Roskill byelection and the electorate will have its new MP declared tonight.
Labour leader Andrew Little has joined the party's candidate Michael Wood at an election function at the Winstone Park Tennis Club, while National candidate and list MP Parmjeet Parmar is holding her event at Frolic Cafe in Royal Oak.
The byelection was held after Phil Goff quit Parliament to become Auckland Mayor.
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 one quarter of National voters opted for Goff.
Wood, a local board member who has run Goff's campaigns in the past, is the favourite to win tonight's vote and his team is cautiously confident.
Prime Minister John Key, who isn't attending Parmar's function, has talked up National's underdog status from the beginning of the campaign, claiming a victory would be such a shock that Little wouldn't survive as leader.
No incumbent Government has won a seat they don't hold in a byelection and there was "relatively little" at stake for National, Key told reporters.
Those comments were dismissed as "trash talk" by Little, who said they were designed to distract from a poor candidate in Parmar.
With Goff's departure National has been given a chance to run Wood close, and if Wood wins by a comfortable margin it will be a boost going into next year's election.
Labour's 2017 campaign chair Phil Twyford as identified Auckland as critical to the party's chances of being in Government next year. Transport is an area Labour believes National is vulnerable, and its promise to fast-track light rail along Dominion Road was a central policy in Wood's campaign.
If Parmar does claim an upset victory National will gain a seat and the next candidate on its list is Misa Fia Turner, who stood in Mangere in 2014.
The extra MP for National would provide more of a cushion when securing the numbers needed to pass legislation.
After NZ First leader Winston Peters' Northland byelection victory in 2015, the number of National MPs dropped to 59, meaning it relied more on the Maori Party or United Future to get a majority.
Most notably, the Government had to go back to the drawing board on planned changes to the Resource Management Act, intended to speed up planning and consent laws.
But the real impact of a Parmar victory would be psychological - losing Roskill would be a major blow to Labour as it enters election year.
• Michael Wood, Labour: A local board member who lives in Roskill South. Says Government has failed to address crime, worsening housing affordability, and congestion. Labour has pledged to bring forward light rail along Dominion Rd.
• Parmjeet Parmar, National: A scientist and businesswoman who entered Parliament on the list in 2014. Parmar wants to make Roskill communities safer, improve transport (she favours buses over the cost of light rail), address housing affordability, and make life easier for small business owners.
• Roshan Nauhria, NZ People's Party: The businessman who was a driving force behind the Balmoral Rd temple is particularly aiming for the Indian and Asian vote, with a core issue of law and order - Nauhria has promised to cut crime by 50 per cent and be a strong voice for Roskill in Parliament, free of party constraints.
• Andrew Leitch, Democrats for Social Credit: The 31-year-old commercial pilot says housing and other social issues show an overhaul of NZ's economic system is needed.
• Tua Schuster, Independent: Says he represents the electorate's poor, who are marginalised by mainstream parties.
• Brandon Stronge, The Cannabis Party: Wants the recreational use of cannabis to be legalised.
• Richard Goode, Not A Party (NAP): Advocates for a transition from democracy to to a "free society based on voluntary cooperation".