John Banks was sent in to save the Act Party - and the latest poll shows it isn't working.

The former Auckland mayor is well behind National candidate Paul Goldsmith, who once wrote a biography of the former Cabinet minister.

Banks was selected after sitting MP Rodney Hide was dumped as Act's leader and dropped as the party's Epsom candidate - even though his electorate success had been the party's political lifeline since 2005.

That lifeline is gone, according to a Key Research poll of 500 Epsom voters commissioned by the Herald on Sunday.

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The poll showed Goldsmith had a big lead over Banks - 32.9 per cent for Goldsmith and 18.9 per cent for Banks. Labour's David Parker rated 4.3 per cent.

The poll - which has a margin of error of 4.38 per cent - also showed many were undecided (41.5 per cent).

Voters had a clearer view of which party they would vote for, with only 10 per cent undecided.

Of those polled, 67.6 per cent said they would vote National while only 5.7 per cent wanted Act. Slightly more wanted Labour at 9.7 per cent.

Banks said Epsom was historically a National Party seat.

"One day every three years they vote strategically and they have been voting for the Act candidate."

He said voters knew bringing in an Act electorate MP would bring in other MPs from the party's list by triggering the party vote.