Thousands of state government communications, including sensitive medical details of West Australian citizens, some relating to the management of the COVID-19 crisism, were published on a public website.
"The breach of confidential data is associated with the use of a third-party pager service," a spokesman for the Premier said.
The breach also impacted St John Ambulance, the Department of Fire and Emergency Services and the Department of Justice.
Technology expert Trevor Long said he was stunned to see highly sensitive medical details "flying around" on an unsecured network.
"It's almost outrageous to think that in this modern age these open and public systems would be used to disseminate this sort of information," he said.
"They urgently need to find a better and encrypted way of transferring this sort of information from one health official to another."
More than 400 webpages – many containing communications and messages between health officials and doctors – were posted to the website. They include details of people in quarantine, including phone numbers and addresses, and how their cases are being managed.