More than 80 per cent of babies are being exclusively breastfed when they leave hospital.
But exclusive breastfeeding trails off drastically soon after. At three months, 42 per cent are fed solely on breast milk, and by six months, just 16 per cent.
Most studies link lower rates of breastfeeding to issues like lower income, lower education levels or lack of social support.
But the researchers argued in the Journal of Pediatric Nursing that the literature has not addressed why many "socially advantaged" women also struggle to breastfeed to six months, even if they have no problems with lactation.
As part of a wider qualitative study, they interviewed 30 women with good education, good incomes and strong social support networks. All were extremely keen to breastfeed exclusively for six months - but just half managed the feat.
Several mentioned that their workplace was not "breastfeeding-friendly" and were reluctant to raise the idea with their employer.
Previous research has shown the most negative influential factor on any breastfeeding duration is a mother returning to work as a full-time employee, while one - a medical doctor - was concerned she would not be able to juggle running a clinic and expressing milk.
Under New Zealand law employers must provide appropriate breaks and facilities for women who want to breastfeed at work.
But the researchers believe some employers are not fully aware of their obligations, or the benefits of breastfeeding - including better staff retention, less sick leave and more productive female employees.
They have called for a Baby Friendly Workplace initiative to be developed, to educate both employees and employers about the legislation.
Other reasons given for starting formula or solids early ranged from "wanting your body back" to being unable to meet the nutritional demands of a rapidly growing baby.
There was a general consensus that breastfeeding advice should be individualised. One mother, Victoria, planned to stick with breast milk till six months but started her boy on solids at five months.
He was "literally reaching for food and growling while we were eating ... it was the main reason why we ended up giving him food", she told the researchers.
Some mums also introduced solids before three months, often under the influence of family members, or if their baby seemed to be "ready" for food.
While the MoH and WHO recommend six months, other studies advocate for a more individualised approach, with each baby fed based on their individual needs, the researchers said.
