Unionised NSW nurses will strike on Wednesday over concerns there aren't enough workers in the health system to deal with everyone needing care.

The NSW Nurses and Midwives Association (NSWNMA) wants one nurse to four patients in general, mental and surgical wards, one nurse per three children in general children's wards, one nurse to three patients in emergency departments and one nurse per patient in intensive care units.

They are also calling for a maximum of four hours patient contact time per nurse or midwife during each eight-hour shift in community health services, NSWNMA general secretary Brett Holmes said.

"The ratios we are seeking will... improve patient experiences and outcomes across the public health system," he said.

The NSWNMA, which represents more than 36,000 nurses and midwives, is holding a special meeting in Sydney during the strike, which will run from 11.30am (AEST) until 1pm.

Members at more than 180 public hospitals and health services will also strike or protest, Mr Holmes said.

While the strike is underway "life-preserving services will be maintained in all hospitals and community health services," Mr Holmes said.

Opposition health spokesman Dr Andrew McDonald, a practising paediatrician, said maintaining the right ratio of nurses to patients is "crucial to improving the quality" of the state's hospitals.

"We know that the areas where these ratios have been previously introduced have seen a dramatic improvement in patient care and nursing satisfaction," he said.

"The O'Farrell government needs to come to the negotiating table and listen to the very real concerns of our nurses and midwives about patient safety."