New Zealand First leader Winston Peters says the Prime Minister is trying to manipulate voters with a series of backroom deals.
John Key announced on Tuesday which parties he could work with after the election including, for the first time, New Zealand First.
Mr Key says he would prefer to work with his current support partners after the election, but could also work with the Conservative Party and possibly New Zealand First.
However, Mr Peters says he will not make any announcements before the election about who he will or will not work with.
"New Zealand First is going to have a long conversation and countless meetings around the country with the public of New Zealand and then consult its membership and its MPs post election - after the people, in a democratic sense, have first spoken.
"That's a critical sequence, the people should speak first not the politicians trying to manipulate the outcome behind the public's back."
Mr Peters says the time for talking about forming governments should be immediately after the election and not before.
ACT supports PM
The ACT Party supports the Prime Minister's decision to keep open the option of deals with New Zealand First and the Conservatives after the election. However, former ACT MP John Boscawen does not believe John Key will need either party.
Mr Boscawen is confident that ACT will survive this election despite the turmoil it has faced since the 2008 election and thinks National will be able to govern with its support only.
However, Mr Boscawen says Mr Key is wise to keep all his options open, even though New Zealand First has been an unreliable partner in the past, and believes it will be very difficult for the Conservative Party to win seats.
Mr Boscawen says Mr Key's endorsement of ACT will help his campaign to replace Epsom MP John Banks as leader of the party.
Labour says Nats more interested in power than people
Labour leader David Cunliffe says the National Party is focused on clinging to office, rather than caring about New Zealanders and John Key will work with anybody.
"This is a time when so many kiwis are doing it tough. Last night we heard mortgage rates could be rising to eight percent. That'll be a real kick in the wallet for our middle income families.
"And what's Mr Key focused on? Not them, he's focused on sticking in power and he'll do a deal with pretty much anybody."
Mr Cunliffe is ruling out any kind of deal with ACT or the Conservatives, but says he would pick up the phone if United Future leader Peter Dunne rang and his door is open to other parties.