Women students are demanding government action after a new survey showed men with the same tertiary qualifications start earning more after just one year.

Women's Affairs Minister Pansy Wong said the survey showed a wage gap opened quickly between graduates with bachelor degrees - up to an average 6 per cent after the first year and up to 20 per cent in some professions after five years.

Ms Wong's ministry carried out the survey, using Inland Revenue data to track the graduates' income.

"The bottom line is that a bachelor's degree held by a woman should be worth the same in the marketplace as one held by a man," she said yesterday.

Sophie Blair, national women's rights officer at the New Zealand Union of Students' Associations, said the situation was a breach of basic human rights.

"It is absolutely outrageous that our female graduates, who do the same degrees as their male counterparts, get paid less right from the start," she said.

"For a long time many have pointed to the high participation of women in tertiary study as evidence of equality between men and women, yet this research shows that in reality there is still a high level of inequity in New Zealand society."

Ms Blair said the Government needed to set out "a real plan of action" to address the pay gap.

Ms Wong said there would be a new 10-year study of up to 6000 graduates to get "new insights" into the situation.

Ms Blair said all that would do was produce evidence of what was already known.

Meanwhile, the Organisation of Economic Co-operation and Development (OECD) has called on countries to tackle the gender equality gap after releasing a report showing women in OECD countries earn on average almost a fifth less than men.

While the wage gap in New Zealand was below 10 per cent, in other countries, including Japan and Korea, men earned about 30 per cent more.

Monika Queisser, the head of the OECD's Social Policy division, said the report noted a growing trend in the number of countries that offer paid or unpaid paternity leave to fathers, but that it was still largely mothers who took parental leave.

"As long as women rather than men take time off work to provide care, there will always be employers who perceive women as less committed to their career than men, and are therefore less likely to invest in female career opportunities and depress female earnings as a whole," Ms Queisser said.

According to the report, women also spent more time caring for children or elderly relatives - at least twice as much as men in almost all countries.

Men universally reported spending more time in leisure activities, but there was wide variation across the OECD. While Norwegian men had just a few minutes more leisure per day, Italian men had nearly 80 minutes more of daily leisure than women.

Across all OECD countries women were more likely to be poor than men, especially from age 66 onwards.

Women aged 66 to 75 were 1.2 times more likely to be poor than the general population, and above age 75, the risk went up to 1.7.