Green MP Sue Bradford has resigned from Parliament after losing her bid to become co-leader of the Green Party.

Ms Bradford will leave Parliament at the end of next month.

Announcing her decision to reporters she revealed that the Greens did not want her to leave.

"The party has begged me to stay."

The decision to leave had been "deeply painful" but she had become disillusioned after missing out on the co-leadership of the party to Metiria Turei on May 30.

Ms Bradford said it was "hurtful" to lose the leadership battle.

In a statement she said: "The Party made a clear and democratic decision, but of course it was personally disappointing and I'm ready for a change."

Shortly after the vote for the co-leadership, in May, Ms Bradford vowed to stay on as an MP.

The 57 year-old has been in Parliament for four terms - stretching over a decade - and said she is proud of haven given a political voice to children and young people, low-income workers and the unemployed.

The reform of Section 59 was her biggest achievement in Parliament, she said, because of the "huge shift" in the way children were viewed.

She said pleased that people could "no longer hit their kids with implements" and praised John Key for not changing the measure after a non-binding referendum showed the majority of New Zealanders opposed it.

If she had decided to wait in Parliament until the anti-smacking debate was resolved she said: "I might be here until I'm 93."

But Ms Bradford said she would not give up on activism:" I want to keep going for it."

She said had been politically engaged since she was 15 and she would stay that way until she was "too old or too sick".

She did not want to end up like many MPs in Parliament whose hearts, she said, were not in it, who were time-wasting and just wanted a salary.

"I'll always be politically active and Parliament is just one vehicle for political change. I'll be going back to the grassroots," Ms Bradford said.

She said she would remain active in community groups and unions.

Green Party co-leader Russel Norman said the party would have liked to have seen her continue as an MP.

"But we also respect her decision to step down," Mr Norman said.

Co-leader Metiria Turei described Ms Bradford as a champion of Green social justice policy.

"Sue has been strong and determined in representing the Green Party's commitment to speak for the under-represented and the most vulnerable," Ms Turei said.

Ms Bradford saw three of her Members' Bills passed into law in the last Parliament which included lifting the youth minimum wage to adult rates, extending the length of time some mothers in prison can keep their babies with them, and amending section 59 of the Crimes Act so that children receive the same legal protection from assault as adults.

Ms Bradford will leave Parliament at the end of next month.

Her replacement is Aucklander Dave Clendon, a sustainable business advisor, who is of Ngapuhi, Te Roroa and Pakeha heritage.