The official results also confirmed that Cabinet minister Paula Bennett suffered an 11-vote defeat by Labour's Carmel Sepuloni in the Waitakere seat. She remains in Parliament as a list MP, but opponents are likely to use the loss to question her mandate.
Bennett said she was disappointed. "I would much rather be standing here with more than 11 votes the other way," she said.
She said the party would seek a recount tomorrow and hoped for a result before Christmas.
She said it was a "cheap shot" to think that the result was a backlash against her or National's benefit reform policies. Labour had run a "negative and vicious" campaign.
"They really mobilised the left vote. They had unions out here 24/7 and stuff has gone on like doorknocking on election day," she said.
Carmel Sepuloni said she felt a sense of relief.
"It was always going to be a feisty campaign. We are different people but we are both feisty women.
"Even if it is 11 votes, me and my team are celebrating a win today. We were always hopeful. We enrolled hundreds of people in the last month so we knew we had a good chance the special votes would go our way," she said.
Sepuloni confirmed her supporters had doorknocked on election day.
"We were allowed to doorknock on election day. We just weren't allowed to tell people who to vote for," she said.
Her win means that Raymond Huo, the lowest-ranked of Labour's successful list MPs, would have to give up his seat in Parliament.
Key said he would be calling for a recount of the Waitakere electorate ballot papers.
The election night dead-heat in the Christchurch Central electorate was broken by a 45-vote majority to National's Nicky Wagner. The win in the traditionally left-leaning seat means one-term Labour MP Brendon Burns will not be returning to Parliament.
The Green Party was the only party to make a gain, with one extra MP bringing its total to 14. Mojo Mathers will be New Zealand's first profoundly deaf MP, and the fifth in the world.
In the referendum, 57.77 per cent of voters chose to keep the MMP electoral system, while 42.23 per cent voted for change.