Family violence and sexual assault data of 4000 Victorians collected over two decades by one of the state's largest health services has been caught up in a data breach.
Monash Health on Friday confirmed its involvement in a breach at external company ZircoDATA, which the health service used to scan archived historical documents.
The breach involved information from family violence and sexual assault support units at Monash Medical Centre, the Queen Victoria Hospital and Southern Health from 1970 to 1993, the health service's chief executive Eugine Yafele said.
Monash Health found out about the breach on March 27 and believes it involves about 4000 people.
Asked why it took so long for the health service to make a public statement about the breach, Mr Yafele said Monash Health had to undergo a forensic investigation of its systems.
"Because of the passage of time and the sensitivity of the information we're talking about, we absolutely needed to be sure that we are able to cross-check and get the right information," he told ABC Radio Melbourne.
"It's taken longer than I would have wanted."
He said data accessed was "a range of stuff", which could include names and addresses of its clients.
Monash Health's systems have not been impacted.
"We are deeply disappointed to be in this position and understand the distress this may cause any impacted clients," Mr Yafele said in an earlier statement.
"We take any matter involving cybersecurity extremely seriously.
"Our team is working tirelessly and diligently to verify those who are impacted and understand what data was compromised."
Anyone who believes they may have been affected is asked to visit Monash Health's website to register for updates or call its cyber breach support line.
The Department of Government Services has been contact with Monash Health as it works to ascertain the extent of the breach.
"The Department of Government Services’ cyber security experts have been working closely with the Australian Cyber Security Centre to monitor and respond to this incident, which involves Commonwealth, state, and territory government entities," a Victorian government spokesman said.
ZircoDATA has been contacted by AAP for a response.
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