The College of Midwives has said it had been "left with no choice" but to take the legal action.
Its statement of claim argues that pay levels for midwives breach gender rules under the New Zealand Bill of Rights Act.
The case is led by prominent lawyer Mai Chen.
A large group, including parents with babies, gathered outside the high court this morning, and received toots of support from passing motorists.
Karen Wakelin, chair of the NZ College of Midwives Wellington region, said it was disappointing legal action had to be lodged, but negotiations over the years had failed.
"There has been very little pay increase...over the years, and more and more [midwives] are leaving practice."
Ms Wakelin said the case was also important for families - and all women.
"This is about providing a service to women - to mothers and babies. But, also, it has far-reaching implications for all women. Because we make up 50 per cent of the workforce, and yet pay parity would show that it is not equal."
Lead maternity carer (LMC) midwives are paid set fees by the Ministry of Health. The College says that since 2007 there have been only two small fee increases, that have not covered inflation.
It argues midwives - almost all female - earn significantly less than male-dominated professions that require similar skills and responsibility.
"Not a case of sexism"
John Key says the "tricky" subject of equal midwifery pay is not a case of sexism.
Hundreds of midwives are launching court action today to sue the Government for allegedly paying them less because they are women.
The Prime Minister told TV3's Paul Henry programme he just "didn't know" whether midwives should be paid more money - and the Government would ultimately be directed by the courts.
"One of the arguments is, well at least the case that people put up, is is it because they are women that the pay is slightly less or is it because that's what the job pays. It depends on which perspective you take," Mr Key said.
"Some professions which are dominated by men have slightly higher pay."
Mr Key said he had not seen a breakdown of the college's claims, but understood the "top line message".
"In the end people are always free to test their rights.
"If they've got a case and the courts agree with them then they'll direct the Government, I guess."
- Additional reporting: NZME News Service
