The WA Opposition wants an inquiry into growing incarceration rates, as workers are put at risk by state prisons bulging at the seams.

A draft report on the efficiency and effectiveness of WA prisons by the Economic Regulation Authority (ERA) came out last week, finding public prisons were not performing well.

The report cited a lack of accountability and transparency as the central reason for prisons not being up to scratch.

The ERA found the average cost of keeping someone in jail in 2013/14 was $352 per prisoner per day, compared to the national average of $292.

WA Labor spokesperson Paul Papalia said the Department of Corrective Services budget blows out almost every year, but much of that money was wasted.

He said the ERA's should be allowed to assess reasons why the prison population was rising.

“Growing prison populations mean government law and order policies have failed - if they were succeeding, we'd be getting fewer people going to prison,” Mr Papalia said.

“What is needed is a proper inquiry into who is going to prison, why they're going there and whether they could be treated more efficiently elsewhere."

A budget estimates hearing was recently told that the number of adult prisoners in WA increased by 17 per cent over the past five years.

In January, the Prison Officers' Union pointed to prison overcrowding as a factor in an alleged assault on four guards.

Corrective Services Commissioner James McMahon says that the current growth rate indicates a new prison could be needed within three years.

Mr Papalia said the government should focus on reducing the prison population, rather than building more prisons.

“Fine defaulters obviously is a clear failure of government, there's a lot of people in prison because they can't pay fines,” he said.

“Also, the Auditor-General recently released a report which shows more than a thousand people last year went to prison for less than four days while they organised bail - that doesn't make any sense at all.

“The Government has to do something else because their current policies are failing.”

Corrective Services Minister Joe Francis hit back at Mr Papalia in a subsequent statement.

“If the ALP believes prisons are full of minor offenders they should tell the public which of these offenders they want to let out,” Mr Francis said.

“This is a complex issue and Labor's fixation with fine defaulters is overly simplistic and misses the point.

“I asked the Treasurer to commission the independent ERA review into WA's prisons ... its findings have strengthened the argument in favour of the current reforms we are undertaking.”