The Productivity Commission report finds that as police detection of crimes improves, the prison population continues to grow, rising by more than 160% per head of population since 1992.

The 516-page report found it costs $111,000 in direct costs each year per prisoner, with another $48,000 per year in indirect costs.

The report estimates that in order to address the growth in prison numbers, Queensland would need to invest an additional $3.6 billion in the next five years to house an additional 4200 prisoners.

The report found Indigenous Queenslanders are three times more likely to be imprisoned than non-Indigenous Queenslanders, women are imprisoned at lower rates than men, but the rate of female imprisonment is climbing and almost half of all prisoners are likely to have been hospitalised for mental health issues or to have had a history of child protection.

Deputy Premier Jackie Trad said the government was implementing reforms including the Action on Ice strategy, the Domestic and Family Violence Prevention Strategy, the establishment of a new Department of Youth Justice, and a new youth detention centre at Wacol.

Ms Trad said that given the cost of keeping prisoners in prison, it might not be the best option for people who repeatedly fail to pay fines, or are repeatedly arrested with small amounts of drugs for personal use.

But the government won't endorse the recommendation to decriminalise marijuana with consideration on the regulatory framework around use, including, for example, the regulation of use in public places.

The report found there were large net benefits (around $850 million) from decriminalising the use and possession of cannabis and there was no evidence consumption went up in places where it was decriminalised (such as Portugal).

A shame the government have pushed this issue into the too hard basket.