National Bay of Plenty MP Todd Muller knows exactly who he will be voting for as his party's next leader, but he's not letting on who.

In an analysis in yesterday's New Zealand Herald, political editor Audrey Young listed Muller as backing contender Amy Adams and not Tauranga MP Simon Bridges.

However, Muller refused to divulge his decision to the Bay of Plenty Times ahead of the party's voting day next Tuesday.

"I have landed on who I will vote for but, from my perspective, when you are dealing with something like leadership there's a full process that needs to be gone through," Muller said.

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"I think it's appropriate that I stand with that and keep it to myself."

Muller said despite this he has already told each contender which way he planned to vote.

Yesterday, Steven Joyce announced his bid for the leadership. He joins Bridges, Adams, Mark Mitchell and Judith Collins in contention for the role made vacant by current leader and former Prime Minister Bill English.

Young speculated Adams and Bridges each had 22 votes from the National caucus already, with Mitchell likely to have nine and Collins likely to have three.

Muller said despite sharing the local region with Bridges, there was no extra pressure to vote for or against him.

"I respect him immensely. I have worked with him over the past four years, as with the others. This is a very strong [group of people] and ... I'm happy with my view of who is the best one.

"It's not stressful. It's very invigorating and exciting. The fact we will have had two weeks by the time Tuesday rolls around to have thought about this, it gives you time to have face to face conversations and form your decision."

Muller said he "thoroughly enjoyed" the leadership race.

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"This doesn't happen often. It's a relatively rare opportunity to spend time reflecting on who's best placed to lead us into the 2020 election."

Neither Bridges nor Rotorua MP Todd McClay could be reached yesterday but it is understood both will vote for Bridges. It was less clear who East Coast MP Anne Tolley or Coromandel MP Scott Simpson were backing.