South Australia’s prison system is set to undergo a major capacity boost with the addition of 350 beds across two Adelaide jails, the government has announced ahead of the upcoming state budget.

Yatala Labour Prison will expand by 312 beds, taking that site’s total capacity to 1,158, while the adjacent Adelaide Women’s Prison will also be boosted by 40, to 316, the government said.

SA Treasurer Stephen Mullighan has described the expansion as the “largest” investment in the history of the state’s prison system, and said $227 million had been set aside in the budget, which will be handed down on Thursday.

Department for Correctional Services chief executive David Brown said there were currently about 720 men imprisoned at Yatala and that the new beds would help ease pressure.

“At different times our facilities will reach capacity and we use all of our beds across the network to accommodate the prisoner population. Yatala is an important hub in the state,” he said.

He said the Yatala beds, to be spread across three new cell blocks, followed an earlier batch of 270 beds in recent years.

“We are committed to our goal of reducing reoffending by 20 per cent by 2026,” Mr Brown said.

“For staff to be able to achieve that, or to help people in custody achieve a reduction in their risk, we need to give them good facilities.”

The government said the work at Adelaide Women’s Prison would begin in January next year, while the work at Yatala would start the following August and “run for a little over two years”.

“[It’s] the single largest investment in our prison system, by dollar and by capacity, in the state’s history,” Mr Mullighan said.

“It’s being done right here at Yatala with a focus on high-security prisoners, making sure that we expand these facilities in three new cell blocks.”

Opposition Leader David Speirs has welcomed the extra beds — but said it was  “nuts” to boost capacity without backing it up with other measures.

“Let’s concentrate on also having a broader prison workforce,” Mr Speirs said.

“How do you expand prisons but don’t have extra prison officers to cope with the new prisoners? 

“I also think we really need a sustained focus on rehabilitative services in prisons — our prisons are rife with drugs, there’s a whole range of other things happening in prison that make people worse when they leave prison than when they went in.”

But the treasurer said announcements regarding new staff would be made closer to the project’s completion date.

“Once we get close to the commissioning of these new facilities that we’re announcing today, in roughly two to two-and-a-half years’ time, then you’ll see further announcements from the government about providing the operating funding and hence the staffing allowance,” Mr Mullighan said.

Mr Brown said he expected the potential growth in the prison population to create a need for about 50 to 70 “additional staff across the network”.