Key Points:

Senior National MP Katherine Rich's says her shock decision to retire from politics is solely due to her desire to spend more time with her children.

Mrs Rich, a shoo-in for a senior Cabinet position if National wins the next election, said the prospect of that had started her rethinking her future.

"While I know I could do a good job, it could not be done by me without having an impact on the lives of my kids," Mrs Rich said.

"I just thought about things over the summer and I really enjoyed my time with the kids. They are at a great age now - they are five and six. They are good healthy and great kids, I am very proud of them and I would like more time with them."

The 40-year-old list MP first came to Parliament in 1999 and has been on the liberal wing of the National Party.

Most notably, Mrs Rich resigned her welfare portfolio under the leadership of Don Brash because she could not back his hardline approach.

Mrs Rich was also the only National MP who publicly backed and voted for the anti-smacking legislation from the beginning to the end.

She came under great pressure from within her caucus and from outside, including threats made against her children.

Mrs Rich said she was not resigning because of a dispute over policy or the direction or the party.

"I firmly believe in the direction John Key is going to take this country as prime minister.

"If I didn't believe in that vision, I would be the sought of person who would stay and fight."

Her political opponents have raised the prospect of National taking a harder line on some issues than they would if the liberal-minded Mrs Rich remained in Parliament.

"There are other people who can fill my shoes, there are other candidates to take my place," Mrs Rich said

Some MPs have expressed bewilderment at her decision.

In the last nine years, Mrs Rich has been there while National went through four leaders, bitter coups and intense policy battles.

Now National has a realistic shot of forming the next government, Mrs Rich is going to leave.

"I don't see the last nine years like that though. They certainly had their ups and downs. It has been a great nine years...I have played my part."

Mrs Rich said she did not fear another three years in opposition and was certain Mr Key would form the next government.

"There is a momentum there that is unstoppable."

There was also no bitterness about Parliament and the sometimes brutal nature of politics - Cabinet Minister Ruth Dyson once called her a "tart" and she received much abuse for her views on smacking.

"This is politics we are talking about. We are not talking about the local Plunket branch. It is tough here, you have to be tough, but that is not something to be negative about. That is just the reality and I understand the reality."

Mr Key said he greatly regretted losing Mrs Rich and he hoped she would be able to contribute in some way to public life in the future

His replacement of Mrs Rich with Anne Tolley to the education portfolio and on the front branch showed that National would still appeal to the female vote, he said.

Mr Key denied that Mrs Rich's departure was due to policy differences or a swing to the right on education policy.

Prime Minister Helen Clark praised Mrs Rich.

"Personally I am sorry to see Katherine Rich leave politics I think she has made a contribution into public debate over education," Miss Clark said.

"I have seen her as someone who is very much at the liberal end of the National Party. The National Party needs such people and I think she will be a tremendous loss."

Miss Clark said she respected Mrs Rich reasons for leaving.