Hi docbert, thanks for your helpful advice. Unfortunately my two tickets are separate - NRT - SYD is part of a DONE4, and the SYD-MEL leg was added later and bought separately. I've called Qantas to have them link the two bookings, which the nice lady on the phone said she had done.

The advice on potentially changing flight times is welcome - I will consider this. BA in the UK is totally inflexible, but I suppose there's no similar shuttle service like this offered.

Thanks again!