Green-thumbed inmates have been caught covertly cultivating dozens of cannabis plants in the nursery of a privately run prison in Victoria.
The state opposition says the incident is further proof the prison system is in crisis under Labor.
Fulham Correctional Centre in Sale is also under the spotlight after the crop of 28 immature plants was discovered in the medium-security prison this week.
There was an escape from Fulham in April and a prisoner was busted keeping a baby snake as a pet in December.
Victoria's corrections commissioner has ordered an investigation into the latest incident in which four cannabis plants were found concealed in the nursery on Tuesday.
A drug-detection dog then sniffed out a further 24 plants during a thorough search on Wednesday.
Victoria Police has confirmed it is investigating.
Corrections Commissioner Jan Shuard admits the latest incident is "embarrassing" and has flagged stripping private operator GEO Group of its licence.
"I'll await the reports and will have to be satisfied they are running a safe environment for prisoners and maintaining the security of the prison," Ms Shuard told 3AW on Thursday.
"That's the fundamental basis of the contracts."
GEO Group has operated the centre since its opening in 1997.
A company spokesman told AAP its own investigation would question prisoners and staff along with a vocational education operator providing horticultural lessons.
He said the plants were all less than 50 centimetres tall.
Opposition corrections spokesman Ed O'Donohue called the episode a farce, arguing the justice system was "simply not a priority" for Premier Daniel Andrews.
The nursery grows plants and vegetables for the prison, the local area's parks and local charities.