'Who is to say where the blast could have gone'
A UNION says the lockdown procedure at the Clarence Correctional Centre may have left some workers at risk.
Construction, Forestry, Mining and Energy Union organiser Dean Riely said local representatives were unhappy with the way the lockdown was put into practice.
Mr Riely said only one section of the jail had been put into lockdown, which could have endangered other workers.
"From what I've been told there was a message found on a toilet door, which matched another message they found which was enough to trigger a response," Mr Riely said.
"From what I've been shown the message said a bomb had been placed under building 10, in male maximum security.
"They evacuated workers from the male maximum security, but workers were kept going in the female maximum security area, despite that section of the site being closer to the possible bomb site than the back of the male maximum security.
"If there had been a large explosive device, who is to say where the blast could have gone.
"Those walls are all made of pre-cast concrete which could shatter and fly anywhere."
Mr Riely said it was likely the bomb threat was nothing more than hoax.
"I've been told the police arrived and have left the scene, which would indicate to me they've done a search and found nothing," he said.
NSW Police were called to the site during the afternoon, but have not commented on the state of the investigation.