This week at least there seems to be a daily flight from SYD to CBR that leaves in the morning or around lunch time. If SQ 231 arrives on time at 11am this flight should be OK, as he can leave HQ from 6am onwards. We arrived on the same flight in December which was during DS and it was due in at 12:15 but we reached the gate at 11:58 - that was important as arrivals after 12 noon must stay in HQ until 6pm. (Or this was the case in December). But SQ 231 seems to always run on time or early so that is looking good. Also if Cooma doesn’t work Wagga and Moruya are both shorter drives than Sydney.
As explained to us by the Police Officer in charge of the hotel that we were to be quarantine in, they go by the time that the plane tyres actually touched down on the runway. This technically means that you have arrived in Australia. Now as to whether he was pulling our legs, I know not!!