"I've always supported the growing of our population, the growing of our hapu and iwi and so I'm certainly not one who's ever believed that we should be controlling people's fertility."
Labour leader David Shearer said it should be extended to all women in need of contraception rather than just beneficiaries.
And Green Party co-leader Metiria Turei said it would coerce women into taking up the offer out of fear they would otherwise lose their benefits.
"The Government has no right to intrude on a woman's choice to have a child. It should get its hands off our wombs."
Ms Bennett and Prime Minister John Key both defended the policy, which Mr Key said was "pragmatic and common sense". Ms Bennett said she would consider it insulting not to offer it, because that took away the choice for women who could not otherwise afford it.
While the idea of extending free contraceptive devices to all women had merit, her immediate focus was on those on benefits.
Mr Shearer also questioned why the responsibility was being put on women rather than men to get contraception.
Ms Bennett said Work and Income already paid for vasectomies for men when requested, and had done so since 1992. However, there were no other long-term options for men - and others, such as condoms, were already free.
She said she did not have a personal view on sterilisation, as it was a personal choice. Work and Income had funded permanent options such as vasectomies and tubal ligations, but they were dealt with on a case by case basis.