"We take threats to the operation and security of our facilities extremely seriously and have detailed protocols for dealing with incidents of this nature", a Corrections spokesperson said.
Adelaide Rd has also reopened after being closed from King St to Rugby St at the Basin Reserve.
At the time of the closure a witness said a white car was coned-off outside a BP service station on Adelaide Rd.
He saw police looking underneath the car and the service station being evacuated.
Police sniffer dogs were brought to the scene.
Metlink earlier told commuters to expect delays and diversions on bus routes 1, 29, 23z, 23e, 19e.
Police are continuing to speak to a person arrested in relation to the threats made.
In another incident, a report was received around 3.15pm regarding an item of concern found in the plaza area off Cable St, near Circa Theatre.
The item was examined and found to be of no concern.