Laurel House manager Marg Dean said child sexual abuse that took place in an institution, such as a church or a school, was the third most common story counsellors at the Launceston-based service heard.

According to the service's latest annual report, 31 per cent of clients said they were abused by a family member, and 29 per cent named a friend or acquaintance.

Seven per cent said they were abused by a stranger, and 28 per cent did not specify.

But Sexual Assault Support Services chief executive Liz Little said she thought the actual instances of institutionalised sexual abuse could be much higher, and she hoped a royal commission into institutional responses to child sexual abuse announced this week would encourage more victims to come forward.

``A lot of the institutional sexual abuse occurred in the past, and we are just really starting to see disclosure,'' Ms Little said.

``I think we have just chipped the surface of it.

``There's a level of legitimacy and a sense of protection and safety in a royal commission, and I think people will feel safer to come forward and speak out.''

Tasmanian Police Commissioner Darren Hine said 23 per cent of sexual assaults reported to police in 2011-2012 were committed at least 12 months before the complaint was made, and one referred to events in the 1950s. 

Ms Little said institutions had been ``cleaned up'' in the past 10 years and instances of sexual assault greatly reduced.

``We are really talking now about a group of people who were assaulted 20 or 30 years ago that are coming forward,'' she said.

Both Ms Little and Ms Dean said the success of the commission would depend on its terms of reference.