Full results are expected by 11.30 pm (Local time).
Earlier opinion polls indicated Key's centre-right National Party is the most popular party, and Key the front-runner to remain the country's leader. But that could be in jeopardy if opposition parties gain more than half the parliamentary seats and form a coalition.
Key's closest rival is David Cunliffe, leader of the liberal Labour Party. His party has pledged to build tens of thousands of inexpensive homes for first-time buyers.
Supporters say Key's party has managed the economy well during six years in office. It has been growing at a 4 per cent clip, while unemployment has dropped to 5.6 per cent.