It was the "most pleasing result" he could have hoped for, but it was not without its danger, Key said.
Key and his National party achieved a unique third-term feat of increasing its vote, rising to 48.06 percent of the total vote cast from 47.31 percent in 2011, Stuff.co.nz reported.
The party won 61 of tthe 121 seats in New Zealand's parliament, up from 59 in 2011.
"The big focus for me over the next three years is that the National (party) keeps it's feet on the ground. I think there's always a risk with third-term governments. They get arrogant," Key said.
He also said there would be "no rapid way back" for former minister Judith Collins, who resigned during the election campaign amid a scandal about her ties to controversial blogger Nicky Hager.
Hager had published a book "Dirty Politics" in August.
The book claims that New Zealand government ministers, including Judith Collins, passed on intelligence and other private communications about certain individuals to controversial right-wing blogger Cameron Slater, known by his blog name Whaleoil.
Key said the scandal that dogged the campaign drowned every party out.
Key had spoken to United Future leader Peter Dunne and Maori Party leader Te Ururoa Flavell and negotiated with them. He will speak to ACT leader Jamie Whyte in the next 48 hours over portfolio distribution.
United Future, Maori Party and ACT are the coalition partners of the National party.
Key has not ruled out giving each party a ministerial portfolio.
Parliament is expected to sit Oct 20, when the new MPs would be sworn-in.