The details of the industrial action are yet to be determined.
The NSW Nurses and Midwives Association (NSWNMA), a PSI affiliate, is calling for one nurse to four patients in all general medical, surgical and acute inpatient mental health wards; one nurse per three children in general children's wards and one nurse to three patients in emergency departments.
Addressing hundreds of nurses in western Sydney, NSWNMA secretary Brett Holmes said Wednesday marked a "pivotal day in the campaign".
"We never willingly walk away from our patients but this state government has left us nowhere else to turn," Mr Holmes told a rally in Sydney.
"So far the O'Farrell government hasn't given an inch."
He said it was unlikely improved staff-to-patient ratios would be part of the next industry award.
"In the next couple of weeks it's likely that you will have new award for 2013 until 2014, and with it a pay increase," Mr Holmes said.
"That award will not contain improvements or extensions to our ratio system because the government will have abused its large parliamentary majority."
Unions NSW secretary Mark Lennon said the government's inaction on ratios was part of a wider attack on the public sector.
"This government treats its employees with contempt and this is a prime example," Mr Lennon told the rally.
The NSW government says it is expanding its nursing and midwifery workforce, boosting frontline services and making sure wages keep pace with other public sector employees.
But Opposition health spokesman Andrew McDonald says nurse-patient ratios had "revolutionised" patient care and should be rolled out across the state.
Please find attached PSI's solidarity letter to the NSW Nurses’ and Midwives’ Association