Some 22,000 public hospital workers are preparing to strike for up to four hours at facilities across NSW over workplace safety concerns.

Some 22,000 public hospital workers are set to strike across NSW over workplace safety concerns with organisers insisting patients won't be at risk.

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

The strike will run for between one and four hours at individual hospitals. There will also be a rally outside the NSW ministry of health offices at St Leonards at 8am.

HSU NSW secretary Gerard Hayes says patients won't be at risk during Thursday's strike action.

"We have skeleton teams within our membership to make sure patients are looked after," he told reporters on Wednesday.

"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 admits there could be some delays, however, for "routine matters".

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

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

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

A NSW Health spokeswoman on Wednesday said an agreement had been reached at the Industrial Relations Commission to avert the industrial action.

"However, the HSU has continued to promote the industrial action and therefore NSW Health has again sought the assistance of the commission to resolve this issue," the spokeswoman said in a statement.

"It is disappointing that the HSU has not acted consistently with the agreement reached in the IRC."

NSW Health said it was working to "minimise any disruption to patients".

Mr Hayes said the strike was required because "40 people a month are seriously assaulted going to work".

"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.