According to https://www.dhhs.vic.gov.au/face-coverings-covid-19
Q. I have a medical condition that prevents me from wearing a face covering, do I need a medical certificate stating I don’t need to wear a face covering?
A. You do not need a medical certificate stating that you have a lawful reason for not wearing a face covering. If you have a lawful reason for not wearing a face covering, you do not need to apply for an exemption or permit.
If you are stopped by police, they will ask you to confirm the lawful reason you are not wearing a face covering.
My comments: The lawful reason you give police if stopped and ask is “I have a medical condition that prevents me from wearing a mask”
The act does not empower police to demand details of what the medical condition is or to ask to see a medical certificate and neither are they allowed to go and ask your Doctor because that would breach patient confidentiality.
Of course I am not advocating you lie to the police, but read between the lines!