Police operations underway at four NSW hospitals

A police operation caused chaos at four NSW hospitals, with at least two being partially evacuated after an apparent bomb threat.

Staff and patients, some in wheelchairs, were evacuated from the top five floors of Randwick's Prince of Wales Hospital, in Sydney's east, today at about 3pm.

Royal Prince Alfred, Royal North Shore and Longueville Private were the other hospitals involved.

A witness at the Royal North Shore Hospital said he was told there had been a bomb scare that forced visitors to evacuate.

Cade Preston, 29, was visiting his grandmother when the warning came for all relatives to leave.

He told ninemsn patients, nurses and other employees of the hospital were allowed to stay but everyone else was ushered outside and told to wait across the road.

Mr Preston said they were told the wait could be several hours.

There were at least a dozen police at the entrance and "heaps more scattered around the hospital", including detectives with crime scene bags, he said.

"Everyone's quite worried and quite scared," Mr Preston said.

Police have confirmed they are investigating threats made against several Sydney hospitals.

A spokesman said detectives were working with the Department of Health to identify those responsible.

A spokesman for the Royal North Shore Hospital's executive described it as a "minor incident".

"For safety reasons we will not be able to let people enter the site - we will let incoming visitors know when it is safe to do so."

Media spokespeople for Royal Prince Alfred and Royal North Shore hospitals and a receptionist at Longueville Private all declined to comment.

Shortly after 4pm, Mr Preston told ninemsn there had been an announcement at the hospital that the "incident was now over".

Police have confirmed some of the hospitals have been cleared, but would not say which ones.

Anyone with information should call Crime Stoppers on 1800 333 000.