National party leader Simon Bridges says despite the loss of three more MPs today, the party is serious and "gearing up for government".

Photo: RNZ, Supplied

Sarah Dowie, Nicky Wagner and David Carter announced in quick succession they will not be running in the September election.

The three politicians are the latest in a string of National MPs stepping down at this election.

Amy Adams, Nathan Guy, Maggie Barry and Alastair Scott all announced last year they would not be running.

But Bridges said this afternoon that he did not think this was a reflection of his caucus' mood or stability.

"It means we're serious, we're gearing up for government.

"You know, as we see people that we farewell, we have the likes of Christopher Luxon, Tim Costley, a very experienced military officer, Nicola Grigg, and many others who are standing so we're ready to govern," he said.

Dowie is currently the MP for Invercargill, but she said she had opted not to seek re-election for family reasons.

"I went to Parliament when Christabel was four and Hunter was two.

"What has become clear is that my children are at a pivotal age and I wish to be 100 percent present to share in their successes," she said.

Last year, police launched an investigation in relation to a text allegedly sent from her phone to former National MP Jami-Lee Ross which said he "deserved to die".

They found the complaints "did not reach the threshold of an offence".

Bridges said Dowie only told him recently about her decision.

"I think for Sarah she took the time over summer. I think the truth is politics takes its toll. If you're a regional member of parliament, in her case Invercargill, she's got two lovely young children and she reflects on those things.

"As she said to me, this is about getting some normality back, and I can understand and respect that," he said.

List MP Carter and Wagner - who was set to contest the Christchurch Central electorate - will both be retiring from politics entirely.

Wagner has been an MP since 2005.

"I have loved my time as an MP and working to make meaningful, positive changes has been hugely rewarding.

"After 15 years, I have decided it is time for me to step aside and support a new candidate to win Christchurch Central," she said.

Carter had been a National MP for 26 years, and has been the minister for agriculture, primary industries and speaker of the House.

Carter and Wagner were two of the MPs mentioned in a phone recording leaked by Ross in which the two were discussing the merits of MPs on the party's list.

Bridges said he did not think that had any bearing on their decisions.

"If you think about that I think both of them had indicated to me in their varying ways that it wasn't their intention to stand again. I really thank them for their contribution," Bridges said.

"David was one of the most competent and fair speakers we've had at Parliament.

"Nicky was a hardworking minister who fought for our most vulnerable and Sarah has been a valuable and passionate advocate for conservation in government and Opposition," he said.

Photo: RNZ / Rebekah Parsons-King

Following the resignations, Bridges announced a reshuffle.

Paul Goldsmith will take over the State Owned Enterprises portfolio, Todd McClay will become the Small Business spokesperson, while Jacqui Dean will cover Conservation.

Gerry Brownlee picks up Greater Christchurch Regeneration, Alfred Ngaro will take on the Disability Issues portfolio, and Anne Tolley will become spokesperson for Veterans.

Jonathan Young will become spokesperson for Arts Culture and Heritage, and Tim Macindoe will take over the Seniors portfolio.

Mr Bridges also announced that Paulo Garcia will become a list-only candidate.

Garcia is the first New Zealand-Filipino MP, and recently accompanied Bridges on his trip to the Philippines.

"He has a great opportunity to get out and about around New Zealand and speak to that constituency in the lead up to the election. Rather than as I suppose a local candidate does, being very focused on a local constituency alone," Bridges said.