The Community and Public Sector Union (CPSU) fears there may be more assaults on prison guards because of overcrowding.
Corrections Victoria has brought in camp beds to be used in communal areas of seven minimum and medium security jails because of a lack of cells.
The State Government says it is a temporary measure, but Andrew Capp, from the CPSU, says it could lead to assaults.
"Our experience shows that it does lead to an increase in prisoner tension," he said.
"Then that in turn leads to prisoner assault on prisoner and prisoner assault on prison staff."
Mr Capp says the situation is the result of an appalling lack of planning.
"The Government came to office on a platform of tougher sentencing laws," he said.
"It introduced those tougher sentencing laws.
"Blind Freddie could see that once that happened that that would lead to a dramatic increase of prisoners in the system."