Part of the Port Augusta Prison has been evacuated after a letter containing a mystery white powder was opened by staff.
The envelope containing the powder was discovered during a routine mail check at the prison this morning.
As a result, the administration building where the letter was opened was evacuated, although the rest of the prison remains unaffected.
Department for Correctional Services deputy chief executive Jackie Bray said the Country Fire Service (CFS) was at the prison and HAZMAT units were testing the powder.
"I can confirm that an unidentified white powder was discovered during this morning's mail process at Port Augusta Prison," Ms Bray said.
"As a precautionary measure, the administration building has been evacuated, and the two staff members involved have been separated."
The two staff were transported to hospital, but assessments found they had no health concerns.
Authorities are yet to identify what the substance was.
In May, bullet holes were found in the front glass doors of the gatehouse of Adelaide's Yatala Labour Prison.
No-one was injured but the union that represents corrections officers said it was the first time the high-security men's prison had been shot at.
A suspicious package was dumped outside the prison's gates in March.
