A trial that restricts visitors for inmates who test positive for illicit drugs is proving successful in a prison in Western Australia's South West, where drug use has plummeted.

Under the 15-month trial, Bunbury Regional Prison staff have carried out random urine tests on all inmates at the facility.

The latest bout of urine testing, conducted in May, showed just half a per cent of prisoners tested positive for drugs — compared with 10-15 per cent previously.

Inmates who test positive for drugs lose contact visits for a period of time and are required to provide a monthly urine sample.

Prison superintendent Kerri Bishop said the program also helped identify inmates battling an addiction.

"[It's about] identifying what they need to help them get through and give up drugs and to be able to sustain that for when they get released," she said.

Assistant superintendent of security Andy Pierre said the lack of drugs in the facility had led to a decrease in violent assaults and bullying.

"We haven't had a staff assault for almost two years," he said

"Our incident rates have remarkably decreased ... and I can't reinforce highly enough that the environment is good, positive and proactive."

Prisoners who test positive for drugs have visiting rights suspended. ( ABC News: Jacqueline Lynch )

A full evaluation of the trial is yet to be carried out but Corrective Services Minister Fran Logan said the preliminary results showed the program had been successful.

He said he would like to see similar trials rolled out in other areas.

"There are a number of programs that we're looking at right now that we can trial in various places across the West Australian prison estate," he said.

Mr Logan has flagged Greenough Regional Prison, in the state's Mid West, as a potential location for another trial.

A formal announcement on Bunbury's results and the future of the program will be made after the evaluation is finished in September.