Health Services Union delegates voted earlier in July to stop work as they fight for increased security at the state's hospitals.

Hospital staff will strike for up to four hours and there will also be a rally outside the NSW ministry of health offices in North Sydney at 8am.

HSU NSW secretary Gerard Hayes said patients wouldn't be at risk during Thursday's strike action, however, there could be some delays for "routine matters".

"We have skeleton teams within our membership to make sure patients are looked after," Mr Hayes said.

"This is about protecting patients in a health setting, it's not about putting patients in harm's way, and so we will be making sure that any patient need is going to be addressed."

Mr Hayes last month described the situation at Bathurst Base Hospital as "dire" and said when it came to violence, local staff endured the same experience as staff at virtually every other hospital across the state.

"Two years ago at Bathurst Base Hospital we had a staff member held at knifepoint. It was dreadful," he said.

Thursday's strike will involve paramedics, security staff, allied health professionals as well as administration and catering workers.

Paramedics say patients won't be charged for treatment or transport during the four-hour strike.

Doctors and nurses won't walk off the job but they support Thursday's action, Mr Hayes said.

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"Forty people a month [across NSW] are seriously assaulted going to work," the union boss said.

"We cannot entertain workplace violence, we have had two of our members stabbed, it's just unacceptable. Everyone has the right to a safe workplace."

The union has called for at least 250 additional security staff in metropolitan hospitals and for them to be given extra powers.