Labour leader Phil Goff has described Mr Key's tacit endorsement of Mr Banks as a rort of the electoral system, a view shared by Mr Parker, who believed the voters of Epsom were over tactical voting.
Mr Parker said the centre-left role in Epsom was not about promoting tactical voting but calling Mr Banks and Act leader Don Brash to account for their record.
"The outcome of Act and Epsom depends on National voters, not Labour voters. The Act Party is at 1 per cent because they are unpopular and it is unpopular because of how it has behaved and because of its record," Mr Parker said.
The Greens' Epsom candidate, David Hay, said Green and Labour voters were interested in strategic voting, and often mentioned a line from Peter Lange at a candidates' meeting in Mt Eden.
Mr Lange, brother of former Labour Prime Minister David Lange, told Mr Goldsmith he was going to "hold my nose" and vote National to cleanse Parliament of Act.
Mr Hay said: "I say it probably doesn't matter that much because Act is so despised by the New Zealand people, they are on 1 per cent and if they win Epsom that might be it, just John Banks."
In the 2008 election, Act leader Rodney Hide was re-elected with nearly 13,000 votes more than National candidate Richard Worth after being tacitly endorsed by Mr Key.
Candidate vote in 2008
21,102
Rodney Hide (Act)
8220
Richard Worth (National)
5112
Kate Sutton (Labour)
2787
Keith Locke (Green).