WA's prison system has reached 96.5 per cent of its operational capacity, with the Government yet to get a handle on the projected impact of burglary laws designed to jail more people for longer.
It comes as the Department of Corrective Services admits two units designed for 128 inmates sit empty at Hakea Prison while it negotiates with the WA Prison Officers' Union on how to open them amid staff shortages.
On March 3 the official prison muster was a record 5085 and by Monday had risen to 5102 - about 150 short of the system's operational capacity of 5253.
The Government last week introduced tough new burglary laws in line with an election promise, mandating minimum sentences of 75 per cent of current maximums for serious and sexual assaults during burglaries.
The laws also close an existing loophole in the "three strikes" burglary laws for offenders over 16 that allows individual charges to be grouped together as one "strike" in court.
Asked what modelling the Government had done on the impact of the laws, Corrective Services Minister Joe Francis said: "We're still working on that."
He said he had no concerns about the size of the prison muster because there were almost 400 beds at Acacia Prison that could be opened in line with demand.
Mr Francis said it was effective to have the prisoner population "just under" the operational capacity because it would be a waste of money to open beds that were not ready to accommodate inmates.
He defended the Government for not arriving at firm modelling despite announcing the burglary laws more than a year ago.
"You don't make decisions on this depending on whether or not you've got space in the system," he said.
"You don't not lock people up because it costs money either, you do what is morally the right thing."
Prison Officers Union secretary John Welch said he doubted more units could be opened at short notice.
Shadow corrective services minister Paul Papalia said it was irresponsible to introduce the laws without also releasing their projected impact on the Budget.