Hi Daniel Jury,
Under the Privacy Act 1968 (1988) – No one is required to tell you or disclose their medical information to anyone.
Also depending on your state, it may constitute a breach under the relevant state Human Rights Commission as well – check under the Dept of Health COVID 19 site.
If no establishment, including police have the power to demand proof of claim (ie medical certificate) then your employer is going to have a push crap up hill battle demanding so!
KYRG have covered this on heaps of videos and podcasts