Being a mother has a huge effect on your pay packet, according to statistics released today showing Kiwi mums earn almost 20 per cent less than dads.

Statistics New Zealand has revealed that mums earn about $23.40 an hour compared to dads, who earn $28.30, a difference of almost $5 an hour dubbed the "motherhood penalty".

The pay gap between women and men without dependent children is 5 per cent.

"The difference between the 17 per cent gap in what mothers and fathers earn, and the 5 per cent pay gap between men and women without children, is significant," said Statistics NZ insights manager Stephen Oakley.

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However, the statistics show that despite the pay gap between mothers and fathers, on average parents do make more than childless men and women.

A dependent child is aged under 15 or under 18 who is not working full-time.

"We found that parents generally get paid more than non-parents," Oakley said.

Mothers with dependent children earn more than $23 an hour on average, almost $1 an hour more than men and $2 an hour more than women without children.

According to international studies, mothers may get paid less than fathers because many women spend time caring for children, which interrupts full-time paid work.

Past New Zealand research suggests the effects of having children on a mum's pay rate may reflect breaks from work and reduced work experience.

Overseas research notes there is also potential for employers to discriminate against mothers, either knowingly or not.

Equal employment opportunities commissioner for the Human Rights Commission, Dr Jackie Blue said the report is an excellent example of why workplaces need to be looking to address what has caused the gender pay gap.

"The results of this research are not surprising, with several studies showing that the gender pay gaps rises markedly when women take on caring responsibilities. For women, it is virtually impossible to recover any ground they lose while taking on caring responsibilities," Blue said.

"This will continue until the culture within our workplaces changes."

"Organisations need to consider whether their own policies and practices are contributing to the gap, and if they are, they need to take leadership and provide flexible work options and supportive environments for women taking on caring responsibilities.

"They also need to develop policies that eliminate unconscious bias, discrimination and negative stereotypes in the workplace, if we are to close the gender pay gap," Blue said.

Blue said men also need to be encouraged to take on more child-rearing responsibilities.

"Providing paid parental leave for fathers would go some way to addressing this."

The Green Party's representative for women, Jan Logie, said the government needs to take decisive action to end the pay imbalance.

"Women have been underpaid for far too long, and we need a strong government that prioritises women getting a fair deal.

"When women are underpaid, it has a real impact on their partners and children, and the life they're able to create together."