Western Australia's auditor-general has found three ministers who withheld information from Parliament due to Cabinet confidentiality concerns, did so unreasonably.

Colin Murphy released a report on decisions made by Education Minister Peter Collier, former health minister Kim Hames and Police Minister Liza Harvey, to withhold details of their portfolios.

In all three cases Mr Murphy found the Ministers had declined to share information with Parliament that was not prepared solely for Cabinet deliberations, and would have been readily available to the public.

"The Ministers could have provided those sections of the plans that can be publicly sourced elsewhere such as annual reports, budget papers, websites and other publications," he said.

"For the content that was confidential and would provide information on the deliberations and decisions of Cabinet this could have, for instance, been redacted with the redacted version of the plan provided to Parliament."

All three cases date back to questions asked in State Parliament in October and December of 2015.

Two of the cases revolved around requests for copies of strategic asset plans (SAP) from the Departments of Education and Health.

The ministers involved, Mr Peter Collier and Mr Hames, declined to provide the information, both claiming the SAPs were part of Cabinet's budget deliberations.

Harvey examined over maintenance backlog of police facilities

The other case examined by the auditor-general stemmed from a question to Police Minister Liza Harvey in December about a maintenance backlog of police facilities.

Ms Harvey replied to the request five weeks later after seeking advice from her department, arguing the information sought was part of her department's SAP and therefore also subject to Cabinet confidentiality.

But in his report Mr Murphy said SAPs were not prepared purely for Cabinet, with much of their content taken from pre-existing documents.

In addition he found the details of police maintenance backlogs were not contained within the WA Police SAP anyway.

He recommended the Department of Treasury review the SAP model it provides to government agencies to clearly identify what parts are legitimately worthy of Cabinet confidentiality.

Mr Murphy also recommended the Department of Premier and Cabinet update the Cabinet Handbook to provide Ministers with better guidance on what can be released publicly.

"Cabinet confidentiality is a long established principle that aims to protect the deliberations and decisions of Cabinet," he said.

"It allows Cabinet ministers to debate freely policies and agency proposals as part of government decision-making.

"While it is important that Cabinet information is kept in confidence; the principles of public interest, governmental transparency and parliamentary access to information are also vital."

WA shadow treasurer Ben Wyatt said the auditor-general's report highlighted the Government's lack of transparency.

"At every turn, the Opposition has tried to get information on behalf of the public of Western Australia," he said.

"At every time the Government relies on this so-called commercial in confidence to hide that information. And again, the auditor-general has called them out.