Women at the agency say they are afraid to assert their lactation rights as bosses make life difficult for women who return to the workforce while their babies are still breastfed.

With women forced to express in toilets and even in their cars outside ATO workplaces, one female tax official has asked the Australian Human Rights Commission to investigate conditions for nursing mothers at the Tax Office.

Guidelines in place at the Tax Office require mothers to get their boss' permission for lactation breaks and some respondents to an internal bulletin board on the issue were scathing of their managers' attitudes.

One woman said she was afraid to approach her manager because she "didn't want to make too much of a fuss as he had already made clear he felt a part-time employee was useless".

Another tax official said she was forced to express in her car in the office car park while another said her husband would turn up at with the baby at feeding time and park in a loading zone while the mother suckled her child in the car.

Although there were some positive reports about managers' attitudes, nearly all the women who responded to the bulletin board said their boss' approach had an enormous influence on their choices.

"I did struggle as I didn't know about the breaks and what alternatives was available to me at the time," one public servant wrote. "So I stopped breastfeeding earlier than expected due to this difficulty."

Another tax bureaucrat wrote of her manager's "contempt" for the process.

"My team leader seemed to have contempt for the whole process and had already made my pregnancy difficult so I didn't push it," she wrote.

Yet another official wrote that she felt forced to wean her son earlier than she had wanted because of the stress of being a nursing mother at the ATO.

"I would often [be] in tears as I had to use the disabled toilet if the first aid room wasn't available and it was always in my time," she wrote.

"Due to the stress with the first time, my son weaned early and [it] was heartbreaking for me."

Now unions want clearly defined lactation rights embedded in the ATO's new workplace agreement, with departmental bosses yet to respond to the claim,

According to the anonymous complaint to the Human Rights Commission, women are being forced to express before or after work or forced by managers to lactate in the office toilets.

"Some [managers] have even suggested if a mother cannot 'properly' return to work, then they should stay home," the complainant wrote.

An ATO spokeswoman refused to answer questions.

"We will be discussing a range of employment conditions as part of the new enterprise agreement process but, as part of our commitment to good faith bargaining, we will not be publicly commenting on the negotiations prior to bargaining discussions," she said.