The Alexander Maconochie Centre.

The Alexander Maconochie Centre. Photo: Rohan Thomson

The former manager of the Alexander Maconochie Centre's crisis support unit says the government's promised secure mental health facility is too small and will come years too late.

Peter Marshall, in a letter to The Canberra Times published today, said the 15-bed centre would ''almost certainly be too small'' and should have opened at the same time as the prison.

He also warned that the centre - designed as the nation's first human rights- compliant prison - lacked the services to meet its obligations under the territory's rights laws.

Mr Marshall's letter was prompted by a recent article penned by senior Canberra prosecutor Shane Drumgold and Wednesday's sentencing of a mentally ill man on arson and harassment charges.

Jeremy Dash-Greentree received a 22-month sentence for setting fire to the front door of a government flat and bombarding a woman with harassing phone calls.

But the paranoid schizophrenic was not required to serve more time in jail after being locked up for more than 18 months.

Mr Marshall, who managed the crisis support unit from its opening until September, wrote that he was ''genuinely delighted'' Mr Dash-Greentree had been released from prison after time spent ''mainly in the crisis support unit which was designed for extremely short stays''.

''There are few more compelling case studies for a secure mental health unit than his experience,'' he wrote. ''That unit, which should have opened at the same time as the Alexander Maconochie Centre, is still years away and, on present plans, will almost certainly be built too small, so that situations like Dash-Greentree's will continue to arise.

''My heart also goes out to his main victim, and I hope she is receiving the support that she also needs at this time.''

The government last year funded the last stages of the design work after a report highlighted the need for a 15-bed centre.

The project has been on the cards for several years, but progress has been hamstrung by cost-blowouts and questions about the demand.

The facility is now expected to be handed over to the government by March 2016, at a cost of almost $24.6million.

The price-tag has fluctuated over the years: first from $11 million, blowing out to $14 million and then being put on hold in 2011 after costs reached $30 million.

Mr Marshall also responded to Mr Drumgold's argument, in Wednesday's Canberra Times, that the Human Rights Act was being used to put defendants ahead of victims.

The prosecutor said the legislation made prison ''much less of a deterrent today, as regular human rights reviews try to make it as close to not being in prison as possible''.

But Mr Marshall said the lack of services on both sides of prison walls left the prison ''unable to comply even with those parts of the act specific to it, such as segregating unconvicted and convicted prisoners''.

He said the proportion of detainees on remand at the prison awaiting trial - one-in-three, and the worst in the nation - was also reason to ''moderate the destructive effect of being incarcerated''.

''Drumgold's office bears some responsibility for the duration of time people spend on remand in custody in the ACT, as do indecisive judicial officers and defence teams,'' Mr Marshall said.