Newly-trained nurse Vanessa is fed up after less than a year in NSW's health system.

She regularly cares for six patients during a shift and sometimes as many as eight, despite union calls for nurse-to-patient ratios of no more than one to four on general wards.

"I've only just started this year and I'm just exhausted already," she told AAP.

"If I knew this before I started my degree maybe I wouldn't be nursing today."

Thousands like Vanessa walked off the job on Wednesday along with midwives, calling for mandated nurse-to-patient ratios and an end to wage caps.

Hundreds gathered in Sydney's CBD, chanting about being understaffed and overworked as they marched from Hyde Park up Macquarie St towards state parliament.

Similar rallies were held across the state, from Albury to Tweed Heads, during the 24-hour industrial action lasting until 7am on Thursday.

The NSW Nurses and Midwives' Association (NSWNMA) says despite evidence of ratios improving patient outcomes in other states, the government has held out in discussions so far.

Association general secretary Shaye Candish says staff are "incredibly burnt out" and at risk of abandoning the NSW system.

"States like Queensland and Victoria are actively encouraging NSW nurses and midwives to go and work interstate because they have provisions like ratios," she said.

The Greens have committed to implementing union supported ratios in full, while Labor has agreed to safer patient ratios without legally mandating nurse numbers.

Opposition leader Chris Minns said he understood the decision by health staff to walk off the job but cited budget constraints when asked about legislating change.

"The budget situation in NSW is tough. We're $180 billion in debt which is the largest figure the state has ever had," he said.

It was better for safer ratios to be achieved through the awards system rather than passing legislation, he said.

"We appreciate that the NSWNMA would rather have it via the legal system ... we think this is a better system to get a better outcome."

Veteran nurse Gaelyn Watson from Prince of Wales Hospital told AAP she had seen conditions decline in wards over the past few years.

"I've never in my professional career seen nurses so dissatisfied," she said.

"It's been increasing over the years till it's got to breaking point."

Ms Watson said patients would continue to suffer without change.

"We're here because we care for the health of our state, clearly more than what this government do," she said.

NSW Health said Wednesday's strike had been staged in defiance of orders from the Industrial Relations Commission.

The current "flexible" ratio system allowed for the professional judgment of nurses and managers to adjust staffing levels to reflect the changing care needs of patients, a spokesperson said.

"Safe and effective staffing involves more than just numbers of staff, it is about making sure there is the right number of staff in the right place at the right time.

"The NSW government and NSW Health have and continue to engage in discussions with the NSWNMA."

Health Minister Brad Hazzard was contacted for comment.