ACT describes the National Party's decision to rule out working with New Zealand First post-election as a "seismic" change on the political landscape.
National made its stance clear shortly after Winston Peters stood down as foreign affairs minister on Friday over an investigation by the Serious Fraud Office into donations to the party.
ACT leader Rodney Hide says the move has left Mr Peters powerless and a vote for New Zealand First is now a vote for a fourth-term Labour Government.
He says Mr Key has put principle before power.
United Future leader Peter Dunne says it's a brave decision and one which creates opportunities for other minor parties.
But he says National supporters may be left scratching their heads if Mr Key's call leaves the party on the opposition benches.
Maori Party MP Hone Harawira on Sunday described Labour as a "coalition corpse" because of its close association with New Zealand First.
But Labour Party president Mike Williams says he does not believe the investigation into Winston Peters is damaging for Labour, as it's an internal matter for New Zealand First.
Meanwhile, SFO acting director Gib Beattie says most of the investigation into donations to the New Zealand First party is still to be done.
A team of four is assessing donations from businessman Sir Robert Jones and the racing industry's Vela family.
A lawyer for Mr Peters said on Saturday the matter could be wrapped up quickly.
Mr Beattie doubts this, but cannot say how long the case will take.
He says he will meet all relevant parties including representatives of the Spencer Trust which handled the money.
Mr Hide expects the investigation will take months and will hang over the election. He says the nature of the complaint means it will take more than five minutes to resolve.
Privileges Committee hearing
The announcement of the investigation was made by the Serious Fraud Office on Thursday.
In addition, Parliament's Privileges Committee has invited Mr Peters, Brian Henry QC and expatriate businessman Owen Glenn to give evidence next Thursday.
The committee is considering whether Mr Peters should have declared a $100,000 donation from Mr Glenn towards legal bills in 2005.
Prime Minister Helen Clark revealed on Thursday that Mr Glenn told her in February that the New Zealand First leader had approached him for money.
Meanwhile, Labour says Mr Glenn has turned down a request to contribute further to the party.
Mr Glenn is Labour's biggest donor, giving the party $500,000 before the last election.
Mr Williams sought further contributions from Mr Glenn in July while on an overseas fundraising trip.
Mr Williams says although Mr Glenn declined his request, he told him he will consider donating to Labour in the future.
Mr Williams also says he had nothing to do with discussions between New Zealand First and Mr Glenn.
He told Morning Report he would have no problem talking to the SFO or the Privileges Committee.