Differences in education, occupation and industry and more women working part-time are often blamed for the gender pay gap, but new research show's that's not really the case.

Auckland University of Technology says those traditional excuses actually make up just 20 per cent of the reasons Kiwi women earn, on average, 12 per cent less than men.

Women's Minister Paula Bennett says the news that 80 per cent of the gender gap is caused by an unconscious bias is "really disappointing".

In her first major speech in the portfolio Ms Bennett has called on employers to hold a gender pay audit.

"It's simply unacceptable that women who are as productive and contribute so significantly to business and the economy are paid less than men," Ms Bennett told the Human Resources Institute of New Zealand on Tuesday.

Researchers say the fact women are now more highly qualified than men should be reducing the gender pay gap.

In response to the research Ms Bennett is calling on businesses to look at whether women are being promoted into positions they deserve and offer clear career progression criteria.

The Human Rights Commission said the research reveals a need for urgent and deliberate action to combat workplace bias, while Equal Employment Opportunities Commissioner Jackie Blue said the results are worse than expected.

"Negative biases and stereotypes are continuing to impact the recruitment, pay and promotion of women in the New Zealand workforce and that's unacceptable," she said.

"We need urgent, deliberate and affirmative action at all levels, to combat the issues they face."

She backed Ms Bennett's calls for employers to regularly audit wages and for them to publish pay gap data.

Labour leader Andrew Little on Tuesday also acknowledged that "decades and decades of discrimination, prejudice (and) undervaluing women" are to blame for the pay divide.

His message to employers is it's "time to get into the 21st century".