"I was told to hide my equipment despite it being in a black bag and when I expressed it was referred to as doing 'that thing'," Irwin said.
"I felt I was doing something unnatural and was asked at least five times when I was going to stop breastfeeding him."
Irwin ended up leaving her job after two months and started her own home-based company importing baby clothes, aptly named Express Baby.
Irwin is not alone in her experience.
A recent study by Women's Health Action found a fifth of workplaces had no support for breastfeeding mums. More than a third of the 75 employers and 484 employees interviewed were unaware of workplace breastfeeding legislation.
Women's Health Action maternal health advisor, Isis McKay, said the study showed an urgent need to educate and support New Zealand businesses about breastfeeding benefits and employers' legal responsibilities.
Some businesses are feeding-friendly and provide private rooms and flexible break times for mums.
Rebecca Lamb returned to her role as marketing manager at ASB head office when her third child, Emma, was 6-months-old.
Her husband brought Emma in so she could breastfeed until she was aged one. For a further three months after that, Lamb pumped breast milk during work hours.
"ASB has its own dedicated breastfeeding room. It was a huge help," Lamb said.
"I'm not sure I could have continued without that, as it's pretty hard juggling breastfeeding and working."
Since April 1, 2009, under Section 69T of the Employment Relations Act 2000, employers have been required "as far as is reasonable and practicable" to provide appropriate breaks and facilities for employees who wish to breastfeed their infants or express milk during work hours.