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The pay gap was closing as more women were going after high-paying career choices and more fathers stayed at home to look after the children.
And there was more sharing of family responsibilities, which would also reduce the pay gap over time, she said.
``I strongly agree women should be paid fairly for what they do and I support the work that's being done to get equal-pay principles into current law.
``But it's an exaggeration to say employers are illegally `taking unpaid labour from women' in the 47 days from today until the end of the year.''
Mrs Nicholls was responding to a media release from the Council of Trade Unions that claimed ``Working for Free Day'', November 14, was no cause for celebration because it meant, on average, New Zealand women started working for free for the rest of the year.
CTU vice-president Rachel Mackintosh said the 13% gender pay imbalance meant the boss was getting free labour out of women in paid work from yesterday until January 1.
Maori women collectively started working for free on October 8. Pasifika women were unpaid for more than a quarter of the year.
``Our law already says taking this unpaid labour from women is illegal and the free ride is about to end.''
There were three parties in Government committed to putting the agreed equal-pay principles into law, she said. A swift and clear legal pathway would be created for women to claim the pay that was rightfully theirs.
The CTU had launched a countdown clock for November 14, 2018 so women could look back together in a year's time and celebrate having fair equal-pay law that was working well for women, Ms Mackintosh said.
Mrs Nicholls had worked in school liaison positions, particularly with careers advisers. She had advocated to both Labour and National the need for professional careers advisers to be available to schools.
``We have a situation now where some careers advisers are incredible and some are not. We need this equalised across the whole secondary school system.''
Boys would also get better advice through an equalised system, she said.
There was an overall earnings gap between men and women but it was not because of some illegal action by employers, she said.
It was a complex area and the gap could stem from career choices being made by men and women and often came about because more women than men took time out during their working lives to raise a family.
``The first reason for the gap is the career choices made by men and women. If you choose to be a child-care worker or a bus driver, you are likely to earn less than an engineer or a construction foreman.''
As more women then men chose to take time off to look after children, it took longer for women to achieve higher-paid positions, Mrs Nicholls said.
Minister for Women Julie Anne Genter said New Zealanders expected women to be paid fairly for their work but the gender pay gap showed women were still being underpaid.
A woman should not be paid less just because she was a woman or she worked in a female-dominated industry.
Even after adjusting for age, education, experience and other variables, there was still an unexplained gap.
``I encourage all New Zealand employers to look at what they can do to understand and close their gender pay gaps.''
Ms Genter said she was committed to closing the gender pay gap, starting with the core public service, and working on pay equality issues for women.