I'm pretty sure you can just walk right to the next gate. If you get lucky and it's actually the adjacent gate, this might take 1 minute. 30 minutes should be fine even with limited mobility. There are QF flights every 30 minutes from SYD to MEL so if you miss your plane you just get rebooked onto the next one. BNE to MEL is hourly most of the day. Virgin is slightly less frequent and less regular. The only exception would be if you somehow need to move between SYD T2 and T3, that would take a bit longer if you can't walk long distances.