Figures from the Australian Institute of Health and Welfare show that  the Deloraine facility had an average of just nine detainees a night in the three months to June.

The number of detainees is sharply down from an average of 27 youths in 2010, and less than a fifth of the centre’s capacity of 51.

Ashley has annual running costs of more than $10 million.

The government says it has cut staffing numbers by about  10 per cent after it was revealed in May that 90 full-time equivalent employees worked at the centre.

Human Services Minister Jacqui Petrusma said  a recent review had found that the state needed  a secure juvenile justice facility, even if Ashley was under capacity.

‘‘Demand from the courts for youth custodial services has declined markedly and, based on this decline, the review concluded a reduction in the workforce could be achieved without compromising legislative requirements, safety and security considerations, or service outcomes,’’ she said.

Ms Petrusma said 9.5 full-time equivalent staff had already been cut from Ashley, mostly through natural attrition.

‘‘Further reductions are part of current consultation with staff and unions,’’ she said.

A review by the previous government into the future of Ashley recommended against its closure, despite noting the facility’s high cost.

The AIHW said the drop in youth detention numbers was driven mostly through a fall in unsentenced detention, which was down from an average of 20 detainees a night four years ago  to about three in 2014.

The state’s rate of incarceration for youths was 1.3 per 10,000 people –  well below the national rate of 3.3 per 10,000 people.

Ms Petrusma said the government was spending $5.3 million on youth justice prevention initiatives.