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

Medical staff and patients, some in wheelchairs, were evacuated from the top five floors of Randwick's Prince of Wales Private hospital about 3pm.

Meanwhile, relatives at Royal North Shore hospital were asked to leave the building, while patients and medical staff were allowed to stay.

Police said they were investigating "threats made against several Sydney hospitals" and were working with the Department of Health to identify those responsible.

The other hospitals involved are believed to include Royal Prince Alfred and Longueville Private.

It is believed all four hospitals received a threatening call this afternoon.

Cade Preston, 29, was visiting his grandmother at Royal North Shore hospital 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.

Patients and hospital staff have been evacuated from Prince of Wales Private. (9NEWS)

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

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

Police said some of the hospitals had been cleared of the threat, but could not confirm which ones.

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."

A police operation is underway at the four Sydney hospitals. (9NEWS)

Patients and medical staff were evacuated from the top five floors of the Prince of Wales Private Hospital. (9NEWS)

(9NEWS)