Mr Williams said Labour took great care on its list.
"So an LEC can sit there and say, 'We want a woman, we don't want any blokes around.' I would take that as discriminatory and I think there are human rights issues there. It will cause considerable debate."
He said it also risked putting male voters off the party and would be an unnecessary distraction for the party when it least needed another internal sideshow.
About 41 per cent of Labour's caucus are women but of its 22 electorate seats only eight are held by women.
Labour leader David Shearer would not comment beyond saying he was supportive of increasing the number of women in Labour. However, he is understood to have voiced concerns privately about it.
It was supported by some female MPs, including Manurewa's Louisa Wall, who said she supported any mechanism to achieve the goal of 50 per cent gender balance, but others refused to comment. The male MPs were less impressed. Shane Jones said it would do little to help Labour get back into Government.
"The last time I checked it was the blue-collar, tradie, blokey voters we were missing. The notion of blocking males is so absurd it's hard to believe it came from people with the interests of Labour at heart."
Phil Goff and Andrew Little both said selections should be based on merit.
"I think that to exclude people because of gender would be wrong. I don't think it will be used much, if at all," Mr Goff said.
The new rules will make Labour's gender equality rules stronger than the Green Party, which has a requirement of no more than 60 per cent of either gender in its top six on the list, and for that ratio to be maintained throughout.
Metiria Turei said Labour's situation was different because it was not a list-only party so it also needed rules for its electorate selections.
National's Judith Collins was dismissive of the proposal, saying it showed Labour women clearly were not confident of being selected under normal processes.
"It is a surprise that they have so little confidence in themselves."
Labour's general secretary, Tim Barnett, said the party used its list ranking to address gender imbalance, but also needed to ensure there was some balance in the electorate seats. Electorates applying for it had to be motivated by the right reasons. "If it is clearly being done solely to block somebody they don't want, then the council can say no."
He said it was up to individual electorates to decide whether to make use of the new rule, should it pass.
Proposed new rules
* Set target of 45 per cent of women in caucus after 2014 election and 50 per cent after 2017 election.
* Local Electorate Committees can ask NZ Council to determine only women can seek selection as candidate.
* List Moderating Committee to ensure correct ratio of women are likely to get into Parliament.
* Moderating Committee also to ensure list represents Maori, ethnic groups, the disabled, sexual orientation and age groups.
