A long-awaited secure unit for mental health patients has opened in Canberra, plugging a gap that has previously seen patients sent to either hospital or jail.

The new facility, named Dhulwa, will house up to 25 patients in need of both acute mental health treatment and secure surroundings.

The $43 million site was years in the making, and will be run with 24-hour clinical support and security.

Matthew, who chose not to give his last name, wound up at Canberra's jail, the Alexander Maconochie Centre, after a psychotic episode brought on by drug use several years ago.

The facility will house up to 25 patients. ( ABC News: Toby Hunt )

He has since pushed hard for the unit to be completed, and said he would have been better served by being treated at Dhulwa.

"Within the Alexander Maconochie Centre environment, I believe there are many individuals that probably could have done with more focused mental health support," he said.

Executive director of ACT mental health Katrina Bracher said before Dhuwala, people like Matthew were either jailed or treated at a less secure hospital ward.

"We have cared for those people in the adult mental health unit with security staff and corrections staff in the unit," she said.

"The need for this facility has been well known over a number of years.

"For many of the people that are here, they will be found guilty by reason of their mental illness, they will have the most severe of the mental illnesses that people can get.

"This is a health facility, it's not a custodial or a judicial facility."

The facility will focus on rehabilitation, and the average stay time is estimated to be about two years.

"It's a really important part of the spectrum of mental health services in the ACT, particularly in the space between justice and corrections," Mental Health and Corrections Minister Shane Rattenbury said.

"We need a secure facility for people with acute mental health problems that really shouldn't be at the jail, but do need to be in a secure environment."