The figure had jumped 2 per cent since 2007, 5 per cent since 2002 and 8 per cent on 1997.
The co-owner of social networking site Mums on Top, Kylie Power, said the cost of living had pushed many women back into the workforce.
"It's quite expensive to have a child. Women are putting children in childcare earlier than ... in the past.
"It's also good for the mental state. Work can be a very nice place to be, a good break to talk to other adults."
Mrs Power returned to work as a primary school teacher in Palmerston North nine weeks after her son was born in 1999, and 11 months after her daughter was born in 2003.
Women then did not always accept working mothers, she said.
"In coffee groups, you'd get a lot of finger pointing. A lot of 'Why are you going back to work?' I'd like to think it's relaxed a little bit now."
In February, Mrs Power and business partner Emma Bettle took over Mums on Top, a private online community where women, and sometimes men, can vent.
"We get everything from 'I'm thinking of having an affair' to 'What sort of cake can I make for my 13-year-old?"'
There have been several conflicting studies over the past two decades of the effects on children whose mothers go back to work in the first year of their lives.
The Institute for Social and Economic Research at Essex University found that children of mothers who went back to work within the first three years were slower learners.
But a University College London study last year found children whose mothers returned within the first year were less likely to fight with their classmates or become anxious than if their mothers stayed at home, partly because mothers with jobs were less likely to be depressed.
In a New Zealand survey this year, 52 per cent of mothers said they felt guilty about their work and life balance and the amount of time they spent with their children.
But more than half of those surveyed said they took time away from their kids to "maintain their sanity".
Claims in a 2010 opinion piece by Jenny Dillon published in Sydney's Daily Telegraph that working mothers should quit complaining because they "have never had it so good" caused outrage from Australian and Kiwi mothers who said juggling parenting and work was no easy task.
The Labour Force Survey showed the percentage of women not employed but looking for work was the highest in 1997 at 4.13 per cent, suggesting workplaces were less flexible then for working mothers.
Dr Ann Weatherall, editor of the Women's Studies Journal and an associate professor in psychology at Victoria University, said the introduction of 20 hours of free childcare helped more New Zealand women get back into work.
"Also mothers are getting older so are more likely to have jobs to return to. There was in that period good government support to have young children in quality childcare, so the availability of that might also be a cause."
She said "it'll be interesting" to see if the trend reverses under National, which in 2010 changed the scheme so it applied only to teacher-led early childhood education.
Balancing work and being a mother
Nurse Emma Buckingham returned to work when her daughter, Molly, was 5 months old.
The 29-year-old is lucky to have a supportive sister who looks after the little girl, now aged 13 months, two days a week during the mother's nine-hour shifts at the Upper Harbour Medical Centre in Hobsonville.
She said she enjoyed being back at work.
"I actually quite like it. It's a bit of normal conversation with adults, a bit of extra money, and a bit of child-free time."
Mrs Buckingham said she found it "a bit of a struggle" in the beginning, but was now in a routine.
"I try to be ready the day before - have her bag packed, her food ready - so we can just bang it all into the car and go."
The mother-of-one filled in for a colleague fulltime for two weeks in September, while her mother and husband Simon's mother shared babysitting duties, which she said would have been hard to maintain long term.
Her days off are spent riding her horses on her parents' property in Kumeu while her mother looks after Molly.
Mrs Buckingham hopes to have another child before Molly turns 5 but is unsure if she would be able to keep working with two pre-school aged children.
