A new prison being planned to cope with WA's increasing jail population could be built through a public-private partnership.

Corrective Services Minister Joe Francis said while planning for the prison had only just started, it could be needed within three to four years.

He said the model used for Acacia Prison had shown that the days of the State Government building jails, in particular medium security facilities, and staffing them with public servants were not the most efficient way forward.

Acacia is a medium security male prison owned by the Department of Corrective Services, but managed by private firm Serco.

Mr Francis said one of the models being investigated for the new prison would see a private company design, build and operate the new jail, with the Government paying them a daily fee for the prison population.

"I think we have an obligation to look at all the options," he said.

"It could be even a build, own, operate, transfer [model].

"At the end of the day what I care about is the safety of the community and what it costs to deliver the prison services."

Mr Francis said while a build, own, operate model would be new in terms of WA's prison system, similar approaches were already being undertaken by the Government in other areas, including for the new Burswood sports stadium which is being done under a design, build, finance, and maintain model.

"These things aren't new and they happen all over the world these days," he said.

Mr Francis has previously said it costs about $150 a day for each prisoner in Acacia, compared to about $300 a day in Casuarina and Hakea, which are both government-owned and operated.

Acacia has recently been expanded to include an additional 400 beds.

WA's adult prison population has risen 17 per cent over the past five years, including an increase of 7.4 per cent last year alone.

As of late last week, there were 5,530 adults in WA's prisons.

But while WA's adult population continues to grow, the State Government denies there is an overcrowding issue.

WA's Inspector of Custodial Services Neil Morgan has raised concerns about overcrowding at WA's main women's prison Bandyup on numerous occasions.