The doors of Adelaide's infamous Z Ward asylum, a one-time home for the state's "criminal mental defectives", will be open to the public on Sunday.

From 9:00am until midday on Sunday, members of the public will be able to walk through the grounds and facilities of Z Ward in the old Glenside Mental Hospital.

Tours of the ward will be conducted by volunteers from the National Trust and Glenside Hospital Historical Society.

National Trust chief executive Darren Peacock said he was very grateful to the new owners of the premises, Beach Energy, for allowing the tours.

It may be the last opportunity for members of the public to see the unique qualities of the site, including the building's polychromatic brickwork and ha-ha wall, before refurbishments begin.

"As far as I know it is the only intact ha-ha wall in Australia," Mr Peacock said.

The ha-ha wall was designed to appear approximately three metres tall from the outside, with a grading slope making an inescapable 10-metre-tall rise from the inside.

During its operation Z Ward housed up to 40 patients with various forms of what would now have been referred to as serious mental health conditions or criminally insane.

The ward was closed in 1973 and pegged for demolition until a local heritage group petitioned for its preservation.

It was used by the Department of Mining for core sample and file storage until 2003, with the building and 2.14-hectare land parcel placed up for sale by the State Government in June this year.

The building is listed on the State and National History Trust lists.