Dame Jenny Shipley, the country's first female Prime Minister and chairwoman of Global Women NZ, said the report showed there was "so much to do" to reach equality.
It was "inexcusable" for a woman to graduate from secondary or tertiary education with equal qualifications to a man and enter the workforce, only to be paid less.
Dame Jenny said women needed to speak up and companies needed to be prepared to tackle any disparity.
Federation of Business and Professional Women president Vicky Mee said she wasn't surprised the country had slipped in the rankings.
"New Zealand has been a leader for many years but we've sort of lost that edge and that push. We are not holding companies to account enough."
She said things were likely to get worse for women in low-earning jobs.
"Things that are worrying are things like the Employment Relations Amendment Act ... I think ... there will be an increasingly big gap between the very low paid in our society and the very high paid.
"And because the majority of low paid employees are women, it will in fact just widen that gap."
Minister for Women Louise Upston said the report showed the country could be doing better.
However, the first place ranking for equality in educational attainment was something New Zealand could be proud of, she said.
Monica Briggs, chief executive of YWCA Auckland, said systemic pay inequity in New Zealand needed to change.
"Women in female-dominated industries getting paid less than work of similar value in male-dominated industries - this is prolific in New Zealand, with women tending to be clustered into a relatively narrow range of occupations that have been traditionally considered women's work and not valued equally."
Read the full report here:
The index aims to understand whether countries are distributing their resources and opportunities equitably between women and men.
The report follows a Court of Appeal decision yesterday, which upheld an Employment Court decision that ruled employers must pay workers in the female-dominated aged care industry wages that were equal to workers in a similar male-dominated industry.
The appeal was brought by Terranova Homes and Care, the workplace of Lower Hutt rest-home carer Kristine Bartlett, who brought the initial case with Service and Food Workers' Union backing.
The New Zealand Aged Care Association had funded the appeal and would not confirm whether a further appeal to the Supreme Court would be launched.
John Ryall, president of the union, said he was "over the moon" with the outcome.
Same skills, two different pay rates for partners
Motueka woman Caitlin Lewis was given a lower paid job than her partner. She believed it was because of her gender, and she felt she had to act.
In 1999 and 2000, Ms Lewis worked for a seafood company as a fish trimmer.
She was employed not long after her partner Brett Edwards - but although the pair had the same skills and background, Mr Edwards was given the more highly paid position of a trainee filleter.
Ms Lewis, now 46 and working as a labourer, said when her first pay cheque arrived and she realised she was being paid less than Mr Edwards, she felt she needed to do something about the issue as it affected not only her, but other women that worked as trimmers in the factory - many of them mothers.
"I'm South African and I was really surprised that New Zealand would let something like this happen.
"It was a huge shock to suddenly be pigeon-holed based on my gender."
She said she thought the problem should be easy to fix, so she looked up who to complain to.
Ms Lewis complained to the Human Rights Commission, arguing that although she and Mr Edwards had identical skills and background, she was given the lesser paid job because she was a woman.
The seafood company denied the allegation but the tribunal found that the job had been allocated on a gender basis and that the company appointed men to be filleters and women to be trimmers.
The decision was appealed in the High Court, which in 2007 concluded the two jobs should not be allocated based on sex.
