Shortly before 11am, a threat involving the hospital was received via a phone call to triple-0 that gave police cause for concern.

A police operation was soon under way, as police rescue and other emergency services crews searched the hospital.

Some lower level ambulatory patients, staff and members of the public were asked to leave the hospital as a precaution.

"Given the nature of the call we received it was quite critical that we enacted the contingency plans as we did in support of the hospital activities, just to ensure that the public, patients and the general presence around the hospital involving all those people, was a safe one," said Wollongong Superintendent Commander Kyle Stewart.

Visitors who were evacuated from the site were forced to mill around the Crown Street and Loftus Street exits. One man had arrived to pick up his wife but was told he had to leave the building.

A woman commenting on the Mercury Facebook page said she was attending an appointment at the hospital when asked to leave by police due to a bomb threat. Police have not confirmed the nature of the incident.

Investigations quickly led to the arrest of a 48-year-old man who is now assisting police with inquiries. Police would not confirm whether the man was a patient, visitor or otherwise.

The all-clear was issued about 40 minutes after the incident began.

"At this point in time our inquiries are looking at ... the identity of the caller and the nature of the information that was provided by that particular caller," Supt Commander Stewart said.

He said the incident was in "no way, shape or form" related to a bomb hoax at St Mary Star of the Sea College last Thursday. Police inquiries are continuing on this matter.

Nicole Sheppard, general manager Northern Illawarra Hospitals Group (which includes Wollongong) said some members of the public were asked to leave the hospital as a precaution.