Unfortunately ST to HK is not possible if you aren’t a HK resident. Even as a resident you would have to enter HK to check in for the next sector and that means 14 days quarantine. Having read around it I think the only way she can travel on your flight is on a commercial ticket where she checks in in OZ and has boarding passes for the whole journey.
That is the sad truth in our new world. I wish I could help and repay some of the courtesy I have received from BA friends and colleagues over the years.