Yatala Labour Prison in Adelaide's north was forced into lockdown this afternoon by two men who dumped a package outside the prison's gates before speeding off.
Police said the men approached the guardhouse outside the high-security men's jail and tried to drop off the package about 12:30pm.
"They were advised this was against security protocols, but the men dumped the bag and took off from the car park in a vehicle at speed," police said in a statement.
A police spokesperson earlier said the bomb squad had been called to investigate.
The bag and its contents were inspected and found to be "non-suspicious", police said.
A roadblock was earlier established outside the prison, which is the biggest in South Australia.
Police said there were no reports of injuries, but inquiries were continuing.
The discovery of a suspicious package resulted in a prison guard being taken to hospital in December.
In that case, the package was believed to be a tennis ball that had been filled with an unknown substance and thrown over the fence.
