By Rebecca Howard 

WELLINGTON, New Zealand--New Zealand's prime minister has vowed to build a coalition rather than govern alone, after his ruling National Party won a resounding victory in Saturday's election.

John Key is set to lead New Zealand for a third consecutive term after official results showed his party garnered 48% of the national vote, and would likely end up with 61 seats in a 121-member Parliament.

A parliamentary majority is unprecedented under the current voting system-implemented in 1996. While the National Party may be able to govern alone with those numbers, Mr. Key said he stood ready to work with like-minded parties.

"In the coming days, I will talk to the leaders of political parties with a view to putting together a broader majority and to ensure a durable and strong government," the former investment banker told Radio New Zealand. "It's a marvelous result that National may technically be able to govern alone, but it's a dangerous position for governments that let it go to their head."

David Cunliffe, leader of the main opposition Labour Party, conceded defeat late Saturday. The Labour Party picked up 25% of the overall vote, according to the Electoral Commission, while the Green Party, always thought to be its likeliest coalition partner, won 10%.

"New Zealanders have chosen to continue and we respect that choice," Mr. Cunliffe told supporters late Saturday night. "Our opponents have built a formidable electoral machine."

The 53-year-old Mr. Key has helped steer New Zealand to a level of prosperity rarely found in developed countries since the global financial crisis, campaigning against a backdrop of the strongest economic growth in a decade.

The extent of National's victory was a complete surprise, said Bryce Edwards, a political scientist at the University of Otago. In the end, however, he said "all of the scandals, all of the colorful and bizarre campaign antics, were not what was important. The economy was the main election issue."

The engine of its growth has been the nation's dairy industry, which is feeding large parts of Asia, and a construction boom fueled by rebuilding after a series of earthquakes in 2010 and 2011. But the incoming government will face new challenges, with growth projected to fall sharply over the coming years as the rebuilding of Christchurch wraps up, prices for New Zealand's main commodity exports fall, and monetary conditions tighten.

Mr. Key has promised business as usual and has said his government would balance the books this year after six years of deficits. "Three more years. This is a victory for those who kept the faith and refused to be distracted," Mr. Key told supporters on Saturday night. Steven Joyce, the National party's campaign manager, said on Sunday: "We are not going to see radical shifts in policy."

Markets are likely to be cheered by Mr. Key's win when they open Monday. "It was a great victory for stability in New Zealand, and markets like stability," said Auckland-based ASB's head of FX institutional sales, Tim Kelleher. And Grant Williamson, a Christchurch-based broker for Hamilton Hindin Green, said: "The market is just going to love that result. It couldn't have been better. It removes all uncertainty."

Mr. Key's victory was stronger than opinion polls were predicting. Meanwhile, Labour's weak showing was the worst since the 1920s, prompting speculation of a possible change in leadership, though Mr. Cunliffe said he had no plans to resign. "I don't believe that rotating the leaders is the key to changing and upgrading our party," he said Sunday in a television interview. "If I did, I would simply stand down now."

The University of Otago's Mr. Edwards said it would be difficult for Mr. Cunliffe to reassert his authority after Labour's poor result, but added that the party still had no obvious replacement.

The Green party's support was little changed from 2011, despite opinion polls predicting it could win as much as 14% of the vote. Green Party Co-leader Metiria Turei said Sunday the party had consolidated its 2011 result and held on to its 10% support even though the country had swung to the right.

The country of 4.5 million people has a multiparty system that allows voters to choose both the party they want in government and the candidate for their local electorate. Governments under New Zealand's proportional-representation system, similar to Germany's, have always been coalitions.

It is unprecedented for any single party under the current electoral system to secure a majority of seats in Parliament, as Mr. Key's party appears to have done from votes counted Saturday. Still, there remain certain special votes to be counted by Oct. 4, though those wouldn't alter the party's overall result.

"There's always a chance you lose one (seat) with the specials," said the National party's Mr. Joyce. "We don't know that yet. The prime minister will want to have relationships with the three support partners we have had and those conversations will take place pretty quickly."

In the current government, National ruled with the support of the ACT, United Future and Maori Party. All three won enough votes to hold seats in next Parliament. The scale of the win rules out the need for Mr. Key to do a deal with New Zealand First, an anti-immigration party that in opinion polls had at times looked likely to become kingmaker.

In government, Mr. Key fought off stiff opposition to the partial privatization of various state-owned assets, and, during the election campaign itself, allegations that his party covertly used a right-wing blog to smear opponents-accusations that cost the country's justice minister her job.

Mr. Key's support also held steady amid allegations by ex-U.S. intelligence employee Edward Snowden that the government had engaged in mass surveillance of its citizens, a charge the prime minister strongly denied. Internet Mana, a party funded by Internet entrepreneur Kim Dotcom, failed to win any seats.

"People could see the country was headed in the right direction and they rewarded us," Mr. Key said Saturday night in a TV interview.

Write to Rebecca Howard at rebecca.howard@wsj.com