The job reduction is part of the state government's move to phase out 20 FTE positions at the centre after a review in May of the facility.

Already 9.5 FTEs have been reduced through natural attrition.

A government spokesman said while detention centre admissions had declined over several years, staff resourcing had not.

"The reduction in the size of the workforce at Ashley can be achieved without compromising legislative requirements or service outcomes," he said.

The centre has been operating at less than 20 per cent capacity with 90 FTE positions.

Community and Public Sector Union secretary Tom Lynch said the government's restructure plan had been worked through with the union and Ashley staff.

"It's not something that we welcome, but there has been acknowledgment for some time that the capacity of Ashley has been far greater than the need," Mr Lynch said.

"We are concerned by the way this restructure has been implemented.

"The savings of capacity of Ashley will go into the government's coffers, rather than back into youth justice."

Human Services Minister Jacqui Petrusma last month said the government would spend $5.3 million on youth justice prevention initiatives.

Australian Lawyers Alliance spokesman Greg Barns said the struggling centre should be sold to establish better rehabilitation options for offenders.

"Ashley Detention Centre now rates as the number one white elephant in Tasmania," Mr Barnes said.

"We've got an enormous amount of infrastructure sitting idle and we are just warehousing young people."

Greens justice spokesman Nick McKim said the decision to cut jobs from Ashley Detention Centre could potentially impact rehabilitation outcomes for young detainees and the local economy.