From the sample of flights I've seen, ex MEL the A220s' favoured routes include to Brisbane, Canberra, Hobart, Launceston and IIRC occasionally to Adelaide,. They make an appearance on MEL-SYD on QF1289 (1930 hours ex SYD) as one example but as a percentage of operated flights on this #1 route, A220s' presence is minimal. They also operate MEL-CFS and MEL-ASP.
SYD-CFS/LST/MCY are some of their routes from Sydney. I couldn't see if they ever travel SYD-BNE.
Seat utilisation on the domestic airline duopoly was already quite high prior to the A220s' arrival.