Invercargill MP Sarah Dowie will not seek re-election as the MP for Invercargill at the 2020 election.

The announcement - timed within minutes of National List MP David Carter and Christchurch Central MP Nicky Wagner officially saying they too will retire from politics at the election - is considered a surprise.

"While this was a difficult decision to make, I feel this is the right decision for me and my family," she said.

Last year, police launched an investigation in relation to a message Dowie sent to former National MP Jami-Lee Ross. They found the complaints "did not reach the threshold of an offence".

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One of the messages from Dowie's phone sent to Ross suggested he should die.

Dowie had already been re-selected as the candidate for Invercargill.

She acknowledged this in her statement this morning.

"Despite being re-selected as the candidate, I have made the decision not to contest the seat."

She cited family reasons for leaving Parliament.

"I am most proud of my children, Christabel and Hunter. I went to Parliament when Christabel was 4 and Hunter was 2 years old. What has become clear is that my children are at a pivotal age and I wish to be 100 per cent present to share in their successes."

She said the new candidate for Invercargill would have her "full support".

National Leader Simon Bridges said all three MPs were fantastic members of National's caucus.

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"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."

Bridges also announced a small reshuffle of shadow portfolios and revealed Paulo Garcia will become a list-only candidate.

Garcia will be one of only three list-only candidates with Paula Bennett and Anne Tolley.

Carter has been a National MP for 26 years. He said his mind had been made up for a while.

"I told both Bill English and Simon Bridges immediately after the last election that this would be my last term."

He also took a dig at NZ First Leader Winston Peters.

"Having endured three robust court battles with Winston Peters, and winning each time, I know well how untrustworthy and difficult he is to work with."

Wagner, Carter and Dowie are the 11th, 12th and 13th National MPs to announce they will be stepping away from politics since the party lost power in 2017.