Hundreds of surgeries are expected to be postponed as more than 50,000 nurses and midwives agitate for better pay.
The 24-hour strike across NSW public hospitals began at 7.30am on Wednesday, despite a previous commitment from the Nurses and Midwives​ Association to pause industrial action.
Nurses and midwives have been calling for a one-off pay bump of 15 per cent after large wage increases for teachers, paramedics and, most recently, police officers.
But such a rise would cost billions of dollars and lift wages "well north" of those in Queensland, Victoria and other states, senior ministers say.
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The strike, the third major stop-work action from the union under the current Labor government, is estimated to force the postponement of 600 to 700 surgeries, according to NSW Health.
Emergency department wait times are also expected to blow out and people with non-life threatening conditions are urged to contact Healthdirect before arriving at hospitals.
Minimal, life-preserving staffing will be maintained in all public hospitals and health services during the 24-hour strike.
Workers previously received an interim raise to cease industrial action until the pay dispute was adjudicated.
Health Minister Ryan Park said he was disappointed the union walked away from its commitment to negotiate before the Industrial Relations Commission.
"We honoured our end of the bargain ... there was an expectation, though, that this industrial action was not going to take place," he told reporters on Wednesday.
The government delivered the union's "big ask" to begin the roll-out of staffing ratios despite the billion-dollar pricetag, but the sought pay rise was not possible to deliver in a single year, Mr Park said.
NSW Health Minister Ryan Park says the government can't deliver the pay increase in one year. (Bianca De Marchi/AAP PHOTOS)
The union rejected an offer for the 15 per cent increase to be delivered over multiple years and to deliver a slightly larger boost for early career nurses, the health minister said.
But the union said the government had not entered negotiations willing to use extra money to fund pay increases.
"Teachers, paramedics and now police have all received historic, once-in-a-generation pay rises," association general secretary Shaye Candish said.
"The decision to give police more than double the offer for nurses speaks volumes," she said.
The government has offered public-sector workers a blanket 10.5 per cent pay rise over three years, including a mandatory superannuation increase, but several unions for frontline workers have negotiated much larger increases.
Mr Park said police agreed to making changes to death and disability insurance, while teachers agreed to restructuring to fund increased wages.
Nurses had not accepted a proposed delay in rolling out staffing ratios, the health minister said, while rejecting characterisations they were being asked to choose improved wages over patient care.
In a fiery question time in parliament, the opposition drew attention to uniformed nurses in the gallery and suggested Labor walked away from pre-election pledges.
"What did you promise?" Opposition Leader Mark Speakman said.
"Other than (lifting) the wages cap, nothing," Premier Chris Minns replied from his seat.