Act is not standing in several marginal electorates - including West Coast Tasman, Waimakariri and New Plymouth - under a quiet deal with National.
Mr Key has said National was campaigning for the party vote only in Epsom - which is Act's lifeline to Parliament as it struggles in the polls.
However he is yet to give a stronger indication to National voters to vote for Mr Banks, saying it "depends if I'm thirsty" on whether he would have a cup of tea with Banks.