Mrs Patterson said that while women taking time out of their careers to have children was a factor in the pay gap, it did not fully explain the divide.
"They would be fine if the research was showing you that the gap occurs during parental-leave absence - but it doesn't. The gap is starting before parental leave.
"There is something happening in that early cohort and then the impact of those gaps over the years becomes too big for organisations to breach."
Another possible explanation of the pay gap is that almost half the women workers in New Zealand are in occupations that are more than 80 per cent female.
Across all sectors for those aged between 25 and 64, for each dollar a man earned, women earned 85.7c - $7,987.20 a year less.
The Government has established a working group to develop a set of principles to deal with claims under the Equal Pay Act 1972 following a landmark case where an aged-care worker who felt she was underpaid because she worked in a female-dominated sector successfully sued her employer.
The woman at the centre of the three-year legal battle, Kristine Bartlett, said: "It's going to give all women a value. Women have been undervalued for too long. It's going to be a life changer in the low-paid workforce, it will give them a bit of dignity and respect to be able to live a decent life."
United States President Barack Obama, in his State of the Union speech yesterday, raised the issue of pay equality in America. "I'll keep pushing for progress on the work that still needs doing ... Equal pay for equal work, paid leave, raising the minimum wage."
Kristine Bartlett, 66: Earns $15.21
For more than 22 years, Kristine Bartlett was paid little more than minimum wage.
She is a caregiver for the elderly and puts up with barely making ends meet because she loves her work and the patients.
But three years ago, Ms Bartlett - who had received a $5.26 per hour rise in more than two decades in the female-dominated profession - decided she'd had enough.
Ms Bartlett, 66, sued her employer, TerraNova, and became the face of a landmark case that tested the parameters of the Equal Pay Act 1972.
"We got a 2c pay rise here and there, then the occasional 5c - bugger all."
Ms Bartlett said all her life, she and her colleagues struggled to live because of how little they were paid - some women had to walk to work in the rain because they couldn't afford a bus fare and others skipped lunch because they were too poor to buy food.
Twenty-two years ago, when Ms Bartlett first started working in the sector, she earned $9.95. She is now on $15.21.
"It's hard, it's strenuous, emotionally draining, tiring, demanding.
"You don't realise while you're doing it because you love your work so much. You sit down at the end of the day and you're just exhausted.
"Then you get your pay at the end of the fortnight and ask yourself how far it's going to go.
"This is just so unfair."
Supported by the E Tu union, Ms Bartlett argued aged care employers and the Government that funds them are in breach of the act because of their failure to address the very low pay rates in the sector.
After three years of court proceedings, they won, to Ms Bartlett's delight.
"It's going to be a life changer in the low-paid workforce, it will give them a bit of dignity and respect to be able to live a decent life."
Her victory opened the doors for court cases from other unions representing low-paid women, prompting the Government to set up a Joint Working Group to determine how to deal with such claims.
TerraNova executive director Terry Bell said he hoped the group's recommendations would include better funding for the aged care sector so they could pay their employees a more liveable wage.
Kirsten Patterson, 42: 'We need to ask for more'
Kirsten Patterson, pay equity campaigner and NZ head of chartered accountants. Photo / Supplied
In her top job at one of the country's biggest accounting firms, Kirsten Patterson has shattered the glass ceiling but she can't foresee women being equal to men in her lifetime.
The New Zealand head of Chartered Accountants Australia New Zealand is fortunate to have never been the victim of workplace discrimination but she can recall a time early in her career when she disadvantaged herself during a salary negotiation.
"Instead of a small pay rise, I negotiated a couple of extra days' leave a year because that was what was important to me at the time.
"But then that set me back because pay rises are usually based on percentage so from then on I was behind and starting off a lower base."
Mrs Patterson said women just needed to ask for more.
"In my view the gap is just not closing quick enough and we need to be arming people with this knowledge and these skills to have the courage to ask."
Mrs Patterson's war on pay equity has been waged on men and women doing the same job for the same pay. She doesn't accept arguments that the pay gap is because women take time away from their careers to have children.
"Research shows that the pay gap starts before that ..."
Another factor of pay inequality is because women are not as good as men at negotiating pay rises. This needs to be tackled with education and training on how to fight for better pay and employers accepting that men and women do not have the same approach.
Businesses also needed to recognise what male and female leadership looked like early on in their careers.
Figures from the World Economic Forum have projected that women will finally receive equal pay 2095 - 80 years from now. She had hoped when her 9-year-old daughter was her age, the pay gap would not exist.
"My mum can't believe I'm still fighting this because she thought they'd tackled the pay equity battle in their time.
"To have seen the results of the statistics from the last couple of months to see that we've slipped is really disheartening."
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It varies between industries, but across all sectors women earn 14.3 per cent less than men and last year it got worse. Women's median hourly wages were 89.4 per cent of men's hourly wages in 2014, but that dropped to 85.7 per cent last year. The last time the gap was this big was in 2006 when it was 85 per cent.
The difference is largest in the community and personal services sector, where women last year earned $7.16 less than men per hour, followed by professionals, where the gap was $6.77.
The smallest discrepancy was among labourers. Male labourers earned just under $3 an hour more than female.
The statistics show the gender pay gap closed most in the years immediately after the Equal Pay Act of 1972, and in periods of economic growth in the late 1980s as well as 1997-2009. The pay gap stalled during tougher times in 1977-84 and 1990-96, as well as since 2009.
"The gender pay gap has been a systemic and enduring inequality for women and is a fundamental breach of human rights," the Human Rights Commission said in a 2011 report on equality in New Zealand.
It is also an economic issue - Goldman Sachs estimated that closing the gender gap would boost the female employment rate, raising New Zealand's GDP by 10 per cent.
There are two fronts on which the battle is being fought: the same pay for the same job and equal pay for equal but different work.
Research conducted by the Ministry of Women's Affairs found graduates entering the workforce were paid similar salaries, but the pay gap increased because women were overlooked for top-paying roles. They were considered likely to take time out of their careers to care for children, were less likely to negotiate and were discriminated against despite legislation.
Minister for Women Louise Upston said an unconscious bias in the way we see and evaluate women and men created invisible barriers for women to enter and progress in their chosen field. Highlighting and educating people about stereotyping was key to addressing barriers that limit women's potential, she said.
"Until women have equal rights, equal choice, equal opportunities, equal expectations, and are valued equally, there is more work to be done."
Another factor is almost half the women workers in New Zealand are in occupations that are more than 80 per cent female, and female-dominated occupations are lower paid, according to 2009 research by the Ministry of Business, Innovation and Employment.
What's being done to close the gap?
One of the problems is many women workers are concentrated in lower-paid jobs, such as caregiving. This factor drove Kristine Bartlett to sue her employer TerraNova Homes and Care and prove female-dominated care work is undervalued and underpaid.
She argued that, if the sector was dominated by men, male and female workers would be paid more. The case was launched with the support of the E Tu union, formerly the Service & Food Workers Union, in the Employment Court.
The decision was appealed by TerraNova but in December 2014, Ms Bartlett won the case at the Supreme Court and any further appeals were declined. The courts determined that to establish equal pay for workers in the female-dominated aged care industry, their pay must be equal to workers in a similar male-dominated industry. The decision has opened the door for workers to take legal action for pay equity.
In September, the New Zealand College of Midwives filed the country's biggest equal pay challenge at the High Court in Wellington, claiming the set fees paid by the Ministry of Health breach gender rules under the New Zealand Bill of Rights Act. It argued that midwives earned significantly less than male-dominated professions that require similar skills and responsibility.
And the country's largest education union, the New Zealand Education Institute, has backed a claim from three female support workers. They are seeking equal pay for their profession, which is managed by the Ministry of Education.
In 2008, a study found there were around 600 support workers whose jobs were roughly equivalent to prison officers in skills, responsibilities and emotional and physical demands. Yet the support workers' top hourly pay was only $19.29 compared with $32.07 for a senior corrections officer.
Last October, the Government established the Joint Working Group to develop a set of principles for dealing with claims under the Equal Pay Act. The working group will give ministers recommendations on how to achieve pay equity consistent with New Zealand's employment relations framework and a well-functioning labour market.
All unions have agreed to put their legal action on hold until the end of March when the group will present its principles.
What are the expectations for the working group?
It is hoped that the group will determine which sectors dominated by men or women are comparable in skill, responsibility and commitment.
John Ryall, National Secretary of the E Tu union, was hopeful the working group and its wider players would reach an agreement for the negotiation of the rate of remuneration for care and support workers.
E Tu has 50,000 members from a range of industries, including engineering, community support, mining and manufacturing.
Mr Ryall believes if the cases cannot be settled, the courts will decide on the principles for negotiating equal pay claims and a rate of remuneration for care and support workers.
"When the courts talk about systemic historic gender discrimination they are not taking this issue lightly, just as historically they have not taken slavery or forced labour or racial discrimination lightly," Mr Ryall said.
Eileen Brown of the Pay Equity Challenge Coalition said the single most effective measure to reduce the gender pay gap was increasing the pay of women in low-paid occupational segregated work, such as the care sector.
"There are tens of thousands of women in low-paid caring work, being paid pathetically and on insulting low wages for the valued work they do. And this is not unskilled work."
The Ministry of Women's Affairs has a goal of economic independence for women, but no plans on how to reduce the gender pay gap, Ms Brown said.
"Inequality in pay is completely unacceptable in 2016. It is outdated, antiquated and unacceptable and when people stop and think about it, why is their daughter getting paid 15 to 20 per cent less than her brother for a job that has similar skills, qualification and responsibilities? It is a no brainer."
Global Women is the secretariat for the recently launched Champions for Change, a group of New Zealand chief executives and chairs from across public and private sectors committed to ensuring diversity in the workplace. Spokeswoman Bridget Dawson said the pay gap figures showed a lack of awareness of women's earning potential.
The University of Auckland is also conducting a large study on inequality in New Zealand. More than 15,000 women aged between 18 and 90 are taking part in the New Zealand Attitudes and Values Study, a 20-year longitudinal national probability study of social attitudes, personality and health outcomes conducted by the School of Psychology.
Using this data, the students will examine how pay differences between men and women, by occupation, have changed over six years. The analyses will reveal whether the pay gap is getting larger and if differences exist in the size of the gap across industries.
The Herald analysed the median wage for a 40-hour week for men and women working in different sectors, using figures from Statistics New Zealand's quarterly employment survey. We have used the median, because it is a better measure of 'typical' pay than average, hourly earnings, and the 25 to 64-year-old age bracket to eliminate early and late career earnings, which are often lower than normal. Debate on this article is now closed.