WA’s Auditor-General says the state’s prisons are “flying blind” on preventing the supply and use of drugs and alcohol in their facilities.

A new report by WA auditor-general Colin Murphy probed the state’s 17 adult prisons run by the Department of Corrective Services (DCS).

It found that the department's policies for minimising drugs and alcohol had expired in 2014, creating the opportunity for “slippages in a whole lot of practices across the board”, Mr Murphy said.

One of the major issues highlighted in the report was the Drug Prevalence Testing (DPT), which the audit found was not being appropriately followed.

“They only test three to four times a year and what we found is a number of prisons have not participated in the tests at all,” he said.

The review found that while 94.2 per cent of prisons completed all tests in June 2014, that number had dropped to just 74.7 per cent by December 2016.

“We know that the problem is worse than the department's records indicate but we have no way of saying how much worse it might be,” Mr Murphy said.

“They're treating non-reports — or failure to conduct tests — as positives, so that's inflating the percentage of prisons without issues and the number of issues is much lower as a result of that.

“The approach they're taking to this lack of reporting is giving them misinformation … it's overstating the positives and it's understating the negative.”

Media reports have claimed that the presence of illicit drugs had created disputes that led to brutal assaults within at least one prison.

The report found poor practices and a lack of security devices where contributing to the issues.

“We reviewed of a sample of gatehouse traffic at three prisons and found 29 per cent of parcels were not inspected and personal rub down searches were not in line with policy,” the report said.

“Practices were worse when staff were required to search other staff.”