The Human Rights Commission says Kiwi mothers deserve more from their employers.
A recent study by Motu Research found a 12 percent hourly wage difference between men and women who are parents, compared to 5.7 percent between those without children.
Researchers also found that hourly rates for mothers decreased by 4.4 percent relative to the wages they could have expected to earn had they not had children.
Lead researcher Dr Isabelle Sin says women who return to work faster after giving birth tend to suffer less with their pay packet.
"Mothers who return to work within six months have a statistically insignificant decrease in their hourly wages, but they do suffer from slower wage growth in the future."
Equal Employment Opportunities Commissioner Dr Jackie Blue says the results of the study are disappointing and action must be taken.
"We need to do better by mothers," she says.
"We say we value the work of raising children - let's back up the lip service with action to ensure we don't penalise them financially."
She says the "mum penalty" is a real and observable problem within the New Zealand workforce.
"The data demonstrates conclusively that being a mum often comes at a significant price, with the gender pay gap exploding when women take a career break."
Dr Blue says businesses will need to rethink the way they handle employees who decide to start families, but also that society at large could do with a shake-up in the division of childcare responsibilities between men and women.
"Why not have pay equity checks for women in the workforce, and when they return after breaks for parenting, to ensure they are not missing out on pay increases?
"We could also take our lead from Nordic countries which actively encourage men to take paid parental leave and share parenting responsibilities."
- Gender pay gap continues to persist
- Men need to take more paternity leave to close the gender pay gap
Along with these recommendations, the Commission says the country needs to make early childhood education and afterschool care affordable for working parents.
"And we need unconscious bias training so that we all, especially employers, recognise and challenge the stereotypes which perpetuate the gender pay gap," says Dr Blue.
The Commission also recommends that businesses which employ more than 100 people publish the pay differences between their male and female employees, so that the gender pay gap can be "forensically examined".