About 600 test results from May 2017 to October 2018 were caught up in the data breach, Ambulance Victoria chief executive Jane Miller confirmed on Friday.
She stressed it did not mean 600 people's results were published on the intranet although "a few hundred" people have already been identified.
"These are people that are in Ambulance Victoria as well as people who applied for jobs," Ms Miller told reporters.
"I can assure you that the team is working very quickly to make sure that we identify everyone today."
Ms Miller apologised for the data breach, saying Ambulance Victoria was investigating although it appeared an inadvertent processing issue was to blame.
"We are seeking to learn about what has occurred so that we can make sure that this doesn't happen again," she said.
Secretary of the Victorian Ambulance Union Danny Hill said he expected the incident would be reported to the Office of the Victorian Information Commissioner.
"The data was still online yesterday and we don't know how long it's been up there," he told AAP on Friday.
The results were taken down after the union raised the alarm this week and an internal audit is under way.
Ambulance Victoria staff are also being provided with wellbeing support.
A Victorian government spokeswoman said the breach was alarming.
"This inadvertent data breach is extremely concerning, and Ambulance Victoria is supporting those affected," she said.
"All documents involved in this incident have been removed from their staff intranet, and Ambulance Victoria is taking a range of measures to prevent any further compromise, including completing an access audit."
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