"There is massive expectation that all of the political parties that are in a position to form a government will act in a mature way. That's really the problem for David Cunliffe."
However, Mr Cunliffe remained adamant that Labour could form a three-party government with the Greens and NZ First, despite the polling.
Internet Party leader Laila Harre challenged Mr Cunliffe about ruling her out directly on the campaign trail yesterday when both attended a union meeting in Auckland.
She said whatever Mr Cunliffe had said, she was certain she would be getting a phone call from him on Sunday. Asked how she could be so sure, she said: "One five letter word: maths."
Mr Key warned it could take weeks to get a clear outcome if the results were close because of the wait for the final count. He was hopeful Mr Peters would not drag talks out further by playing parties off against each other if he was kingmaker - it had taken almost three months in 1996.
Mr Cunliffe also said he hoped to put a Government together relatively quickly.
"I am sure we are going to get the new Government up and running before Christmas. We want to get Parliament called back before the Christmas break so we can get the work started."
Mr Peters said speculation he would drag out negotiations were "falsely scaring the public when there's no need to". He said it had taken less than three weeks in 2005.
Neither Mr Key nor Mr Cunliffe would be drawn on what positions might be up for grabs to secure Mr Peters' support. Mr Key would not rule out taking him on as deputy or offering positions such as Speaker or Minister of Foreign Affairs.
Meanwhile, almost a fifth of those likely to vote in this year's election have already done so through advance voting.
The Electoral Commission yesterday confirmed 434,197 advance votes had been cast so far. Assuming a similar turnout this election to that in 2011, that works out about 19 per cent of the total vote.
The figure has already easily topped the 334,558 advance votes or 15 per cent of the total cast in 2011, the first election held under rules that meant anybody could advance vote.