Compulsory therapy for all sex offenders in jail was a pre-election pledge affirmed by the government this week.

Details are yet to be realised but already questions are being raised about the program's feasibility.

Criminologist Lorana Bartels said introducing mandatory jail terms and axing suspended sentences was going to strain the state's corrections budget.

Dr Bartels said if release from jail was conditional on the program being completed then unco-operative sex offenders could remain behind bars.

Tasmania already has about about 20 prisoners indefinitely detained.

However, unlike other states, Tasmania's indefinite sentence provisions only apply to violent offenders.

Professor Stephen Smallbone, of Griffith University, said post-sentence detention was becoming more common in some states and Tasmania may follow.

"If people are seen as risky offenders and they're refusing to participate in recommended programs, they're really unlikely to get parole and then you have issues around post-sentence detention," he said.

Professor Smallbone believed mandatory treatment for all sex offenders was problematic because there was "enormous variation" within that group.

Jail also delivered poorer results compared with community-based treatment.

"There's a lot of pressures within jail, like associating with other offenders ... so trying to reveal personal things in those sorts of circumstances can be dangerous," he said.

The Justice Department is rolling out a much- needed community-based treatment program.

A government spokeswoman said the government stood by its policy and compulsory treatment would be introduced in its first term.

In 2008 the Tasmanian Law Reform Institute advised against introducing indefinite sentences because a dangerous criminal declaration was effectively the same.