Green MP Sue Bradford's sudden retirement from Parliament yesterday reflects little credit on her party. With typical candour she declares her decision was prompted by the party's co-leadership election a few months ago, which she lost to a young Maori, Metiria Turei.

Normally this would sound like sour grapes but whatever one thinks of Ms Bradford's politics, she does not seem to suffer from wounded pride or excessive self-importance. She is remembered for the indignities she was willing to suffer in the years before entering Parliament when she was pictured in every small protest sit-in, usually being carried away by the police.

It is one of the strengths of MMP that several seasoned agitators have been able to come into Parliament on the lists of smaller parties. There, faced with the challenge of hard policy and responsible politics, they have either risen to the task or retired as soon as they decently could.

Sue Bradford was definitely a riser. Conspicuously caring and conscientious, she was constructive in discussion of social issues and won respect across the House. She also has the rare distinction for a non-minister of piloting three bills into law. One of them abolished a statutory minimum wage for youth, lifting it to the adult minimum. Another extended the period some imprisoned women can keep newborn babies with them. The third was the anti-smacking bill.

When Jeanette Fitzsimons relinquished the female co-leadership this year Ms Bradford was clearly the strongest candidate to replace her, and she knew it. Ms Turei was barely known outside the party and Sue Kedgley, another previous campaigner who has found her feet in Parliament, seemed not to be interested.

So why did Ms Bradford miss out? It is reasonable to conclude the Greens wanted a different face. They are a party sensitive to demographic character, as evidenced by co-leadership from different genders. Ms Turei offered youth and ethnic diversity. In the four months since her election she has not shown much else.

A party that puts appearances before substance is making difficulties for itself. Disheartened, Ms Bradford says Parliament is not the only vehicle for political change. "I'll be going back to the grassroots."

She has come too far, won too much parliamentary respect, to return to protest arrests. Her credibility in social policy should be lent to a needy cause.