You may have to look into whether or not she can "enter" Hong Kong to check-in again for the next flight.
Unless they can check her in all the way to the UK with staff travel, I think you may have an issue.
1) Does she hold a HK ID card to be allowed into HK?
2) Doubt they will allow someone to enter HK and not do the mandatory quarantine before being allowed to check-in for the next flight
3) Will they allow her to board the flight from SYD if they cannot check her through to the UK
Hopefully you can find a solution,
Good luck