As of today, New Zealand women are effectively working for free until the end of the year because of the 8.2% gender pay gap.

Based on that gap, the average Kiwi woman is paid 336 days for the equivalent of 366 days for men, according to the Ministry for Women.

To help reduce the inequality, the ministry has launched an online tool for businesses to calculate their difference in gender pay.

"The old adage of what gets measured, gets done is quite powerful," Ministry for Women chief executive Kellie Coombes said.

"We really hope that businesses understanding their gap will really give them the information and push they need to then take action to close it."

The gender pay gap is slightly down from a year ago, but it hasn't shifted much since 2017, when it was slightly over 9%.

According to the ministry, the median hourly pay for women is just over $32.08, compared to $34.95 for men.

But when it's broken down into ethnicity, it's clear who's being disadvantaged.

Women of European background earn $33.70, whereas Asian women are paid $30, wāhine Māori even less on $29.71 and Pasifika women lower again on $29.

New Zealand has 3000 businesses with more than 100 employees, but just a fraction measure the difference in gender pay.

"On the pay gap registry, we've got 120 who are measuring and reporting their pay gaps," Mind the Gap co-founder Dellwyn Stuart said.

She said the online calculator provides a big opportunity to get more businesses on board, but it alone won't eliminate the disparity.

"We will need legislation absolutely."

She said there needs to be mandatory reporting of pay gaps to properly shift the dial.