Nurses and midwives are launching legal action against the NSW government over staffing ratios they say are putting patients at risk.

The NSW Nurses and Midwives' Association revealed plans on Wednesday to file the case in the Supreme Court which accuses the government of repeatedly breaching award conditions.

It says widespread non-compliance with agreed staffing levels has deprived patients of more than 100,000 hours of care at multiple public hospitals.

General Secretary Shaye Candish says the shortages constitute a welfare risk.

"We are talking about hundreds of thousands of nursing care hours not provided on general medical and surgical wards, meaning patients may have missed timely care, such as blood pressure checks, wound care or showers due to inadequate or unsafe staffing," she said.

Health Minister Brad Hazzard accused the association of using members' money to run a political campaign against the government.

"They know damn well the opposition is not planning on giving them ratios," he said.

Nurse numbers are calculated under Nursing Hours per Patient Day (NHPPD) within the Public Health System Nurses' and Midwives' (state) Award.

Nurses and midwives have held repeated industrial action over the past year calling for mandated "safe" staffing ratios, along with better pay and conditions.

NSW Health defended the NHPPD as a "flexible" tool for hospitals to adjust staff numbers and said compliance was improving, citing a record number of nursing and midwifery graduates this year.

Both major parties are taking policies to the March 25 election they say will increase the number of staff on wards, while citing budget limitations in directly meeting the union's standards.

Opposition Leader Chris Minns has promised to replace NHPPD with enforceable minimum staffing levels, beginning with emergency departments, which the association previously welcomed as a step in the right direction.

The position is a shift from Labor's previous support of legislated mandated nurse-to-patient ratios, to a different form of award agreement.

In the most recent budget, the coalition allocated $4.5 billion over four years to recruit 10,148 full-time equivalent staff to hospitals and health services, including nurses and midwives as well as doctors, paramedics, pathologists and scientific staff.

The union says it has evidence of inadequate staffing at major public hospitals including Royal Prince Alfred, Gosford, Wollongong, Westmead, Liverpool and Nepean.

The association identified Gosford Hospital on the Central Coast as the worst offender for staffing breaches, with 777 award contraventions over four years.

"The 1484 contraventions we are filing today are just the tip of the iceberg," Ms Candish said.

"If anything, we have been conservative in this prosecution and have not included a large number of other hospitals that also breached the award repeatedly."