"In these tough times, every dollar counts. We can't afford to wait any longer.
"Requiring big employers to report pay gaps can help reduce child poverty and help end discrimination that impacts on the aspirations of Māori, of Pasifika; of other ethnic groups."
According to analysis of public pay gap reporting in seven countries, mandatory reporting can reduce gender pay gaps by between 20-40 per cent.
Applied to a New Zealand context, it means a woman earning the current median wage in New Zealand ($26.37) could receive $12.80 a week more ($35.77).
"Overseas experience shows that making it mandatory for businesses to report their pay gap has resulted in, not only closing the gap but quickly having a positive impact on workers' pay packets, once the legislation is announced," said Dr Jo Cribb, co-founder of the MindtheGap campaign.
While an increase of up to $35 may not sound like a lot, Cribb said, it could make a difference to the lives of low-paid employees.
"$35 dollars a week will help with this winter's heating bills or buy twelve litres of petrol. We literally can't afford to waste any more time addressing this issue."
Signatories to the open letter include the Human Rights Commission, E tū, New Zealand Council of Trade Unions, National Council of Women, CAANZ and Sports NZ.
"As a country, we can help improve the wellbeing of our working people with simple steps such as pay gap reporting. We look forward to working with the Government to see this become a reality," said New Zealand Council of Trade Unions secretary Melissa Ansell-Bridges.
So far 55 out of an estimated 5000 companies with more than 50 employees are reporting their gender pay gaps on The MindTheGap campaign's Public Pay Gap Registry, with seven of those also reporting their Māori and Pasifika pay gaps.