Women's Affairs Minister Louise Upston is the first National MP to sign a pledge to close the gender pay gap.

The minister said this afternoon she was proud to sign the pledge, which is part of a union-led "We're relying on you" campaign for pay equality.

Another 24 female MPs and eight male MPs have also signed up, saying they believe people should be paid according to their job, not their gender.

The three unions running the campaign held a picnic on Parliament's lawn this afternoon to mark "Equal Pay Day".

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Campaigners said that because New Zealand women earned 14 per cent less than men on average, they are effectively working without pay from today until the end of the year.

"I ask you, would you feel short-changed if 2015 was to finish today?" New Zealand Nurses Organisation industrial services manager Cee Payne told a crowd of around 200 people.

Ms Upston said full equality was still some way off in New Zealand.

"Until women have equal rights, equal opportunities, equal expectations and are valued equally, we have work to do.

"And we still have got a lot of work to do."

Employers and unions agreed last month to set up a working group to develop principles on pay equity in all sectors of the economy.

Green Party MP Jan Logie said the next step was for the Government to set a date for reaching pay parity in the public sector.