Four cars have been destroyed by fire at the Yatala Labour Prison staff car park in Adelaide's north, in what is being investigated as a suspected arson attack.
The early morning incident has prompted renewed calls for enhanced safety at the site from the union representing prison staff, which said its members felt "absolutely outraged" by what had occurred.
The Public Service Association (PSA) also said two staff had encountered a person wearing a balaclava when they arrived for work this morning, but that the person fled the scene.
"We've got major security concerns and safety concerns for the Yatala carpark being wide open like it is," PSA SA vice-president Craig Bates told 891 ABC Adelaide.
"We have been calling on the department to erect a fence."
Police are investigating the incident and have described it as "suspicious", and said emergency services were called to the site on Peter Brown Drive at Northfield about 4am.
They said two Mitsubishi station wagons and two Toyota sedans were completely destroyed.
Police have asked anyone who witnessed suspicious behaviour in the area or has relevant dash cam footage to contact Crime Stoppers.
Department for Correctional Services chief executive David Brown said there was CCTV of the incident, but said he would not go into the details of what it showed.
But he said he had met with management at the site this morning.
"We've had a critical incident at the site," Mr Brown said.
"It's obviously a live and very active police investigation and we are always looking at ways we can enhance safety for our staff, including the safety of the carpark area.
"It was an outrageous incident this morning — four government vehicles were set alight and have been written off by this act of arson. I'm very concerned about the nature of the incident."
The incident has been described by police as suspicious. (891 ABC Adelaide: Bobby Macumber)
The PSA said it had raised concerns five months ago, saying that "inadequate security was putting staff at risk".
The union said it wrote to the department in May urging security improvements including "an extension of the perimeter fence and 24-hour protective security officers", but that those requests were rejected.
"Only a number of months ago, a stolen vehicle was driven through that car park with people wearing masks and they were later found to have a firearm," Mr Bates said.
"There have been other issues with vehicles being damaged and stuff like that too.
"A fence is a bit of a no-brainer and it's high time it happened. To be honest it should have happened ages ago."
The department CEO said the incident would be reviewed, and did not rule out the installation of a new fence.
"Every option is on the table including whether we can partition or fence off access to the car park, but also recognising that Yatala Labour Prison is a large prison and it has a lot of interface with the public," Mr Brown said.