My research is shallow, and the number five years stale:
https://profile.id.com.au/australia/dwellings?WebID=250
Nearly 440 000 residential flats in greater Sydney, in 2016. Extrapolation suggests over 1M of the city’s population are thus contained.
If 75% of those are older buildings with insufficient funds to implement and maintain an “interventionist” Covid management strategy - as in, simply relying on people doing what are considered the right things - that’s a substantial amount of the city’s population at imminent risk of contagion.