"We don't think there is any chance of Garth McVicar winning the seat but we think there is a risk he could take votes away from Wayne Walford. Overall we know we're in for a tough fight there."
Asked if he thought the party had put enough effort into retaining the seat, currently held by retiring MP Chris Tremain, Mr Key said he believed it had.
"We've been up against a long-running campaign from Stuart Nash. He's spent a lot of time working in the electorate [and National have had] a change of candidate with us losing Chris Tremain," he said.
"Mr Nash has had the amalgamation issue to run on and we're suffering a little from vote splitting. So it's definitely still winnable from our point of view but it's a much tougher fight than it probably should be."
Mr Nash's anti-amalgamation campaign has highlighted the fact National changed the Local Government Act in 2012, removing the power of individual jurisdictions, such as Napier city, to opt out of proposed council amalgamations. Labour has promised to reinstate that power but Mr Key said National would not.
"If you look at it, it's still very difficult for amalgamation to take place and the Local Government Commission is considering a range of proposals that they have around the country," he said.
"We think overall the Auckland super city has been successful, and while we totally agree it's the local community that should decide what the structure of their local government is, we'd be reluctant to repeal the changes that we made in 2012."
Mr Key's stop in Havelock North's village centre attracted a pro-National crowd of about 200, who he addressed briefly before mingling for handshakes and selfies.
"We've got to get out there on Saturday and win that election and win it well," he told the audience.
"Make sure you get out there, get your friends out there, don't second-guess the polls."
In the Tukituki electorate, which includes Hastings and Havelock North, cabinet minister Craig Foss is defending a 9660 vote majority from the last election against a challenge from Labour candidate Anna Lorck.
Mr Key said he was "very confident" Mr Foss would retain the seat.
In Napier he shook hands with a crowd of about 50 outside Napier's Masonic Hotel before flying to Auckland for a television interview last night.
A team of Conservative campaigners wearing their party's livery - including Mr McVicar and Tukituki candidate Stephen Jenkinson - gatecrashed Mr Key's brief Napier walkabout. Mr McVicar's campaign manager, Louise Parsons, even convinced the Prime Minister to pose for a selfie with her.
But Mr Key made it clear that agreeing to a photo was as far as it went in terms of pre-election friendliness between the two parties.
He reiterated his stance that National did not believe it was appropriate - in Napier or elsewhere - to do a deal allowing easier entry into Parliament for the Conservative.
"The conservatives really need to win 5 per cent. If they don't and they come in too much on National's coat-tails they probably won't build a long-term parliamentary force," he said.
"I know it sounds like we're making life difficult for them but the reality is they've got to get there under their own steam otherwise we're not really helping them."
Earlier Mr Key also visited Dannevirke, where crowds thronged High St as he pulled into Shires Fruit and Vege Market for a meet and greet opportunity.
"You've got to give the man credit because despite all the s**t thrown at him during this campaign, he's never buckled," Joe Todd said. "Key told me he's made of hard stuff."
For National's Wairarapa candidate Alastair Scott it was birthday time, but he had only one message on his mind.
"We've got to get everyone out to vote," he said. "I trust the people of New Zealand to vote the right way, because, at the end of the day, you get what you voted for after Saturday night."
Mr Key was asked by media what he would do for retailers in rural towns such as Dannevirke where many High St retailers were struggling.
"The most important thing is to build up the infrastructure to enable people to live and work here," he said. "Extending our ultra-fast broadband network is hugely important because development in areas like this which are the food bowl of the country is essential."
Mr Key also assured one member of the public the National Government was working hard to reduce youth suicide.
"We're putting $64 million into youth mental health because this is a serious issue," he said. "But youth suicide rates are slowing starting to fall."
A small group of anti-fracking protesters were also present.
