National and Act are working on a deal under which Act would not stand candidates in marginal seats, including New Plymouth and Waimakariri, to increase National's chances of winning them.

The deal would be in return for National again gifting the Epsom electorate to Act by having its new candidate Paul Goldsmith campaign only on the party vote.

It would give National a greater chance of wrestling Canterbury's Waimakariri electorate from Labour's Clayton Cosgrove and holding on to New Plymouth against a strong challenge from Labour's Andrew Little.

In 2008 National signalled Epsom voters should continue to support Act's Rodney Hide, giving National a coalition partner.

A similar deal this election would see Act return with four or five MPs on current polling without having to break the 5 per cent threshold.

Neither side would confirm the deal yesterday, saying discussions were under way.

Act leader Don Brash said there was "no formal agreement of that kind at this stage" but he and Mr Key wanted "a strong centre-right Government ... and everything we do will be aimed at that objective".

Mr Cosgrove held off National's Kate Wilkinson in Waimakariri in 2008 with a majority of just 390 - less than the 1717 votes won by Act's candidate.

National's Jonathan Young took New Plymouth from Labour's Harry Duynhoven with a slim majority of 105, which would be bolstered by the 455 votes Act's candidate got.

The Herald understands Act and National have discussed a similar deal in other marginal seats, including Maungakiekie, won by National's Sam Lotu-liga with a margin of 1942 votes.

Act's new candidate in Epsom - John Banks - said while he was not privy to such discussions, "Waimakariri would have been held by a National Party member of Parliament today if it wasn't for the Act Party candidate last election".

"I don't make those decisions. I'm not on the campaign committee and I'm not on the Act board so I can't speak with any authority but would it make sense, for instance, to lose the seat of New Plymouth to Andrew Little, a future leader of the Labour party, likely to be the leader of the Labour Party post the election, to lose that seat held by Jonathan Young because we put up an Act candidate?"

Mr Banks indicated Act would consider a more general policy of not standing in marginal seats. "I would not want to dislodge any high-quality National Party MPs from their marginal seats by way of putting up an Act candidate."

Mr Goldsmith yesterday made it clear his priority was to campaign for the party vote in Epsom.

* Won in 2008 by Labour's Clayton Cosgrove with a majority of 390.
* Act's Aaron Keown polled 1717.

* Won by National's Jonathan Young with a majority of 105.
* Act's Chris Albers polled 455.