The difference here is that the first point of transmission was not in hotel quarantine but someone who was infected and came from South Australia then travelled into Victoria and infected others who also travelled widely. That creates a nightmare scenario that wont be fixed by just contract tracing. The lockdown is necessary to prevent rampant spread of the virus and then contact tracing can work to identify those at risk and have them confined. With 650 contact locations contact tracing on its own faces an impossible task.
These new virus strains are more infectious and you will find that its likely that other States will be in the same dilemma if someone with the virus escapes the net and travels widely as occurred with the one that came from South Australia. The only way to avoid this is to break the back of the spread with a lockdown allowing contact tracing to do the rest. You mention NSW and I have no doubt if the same degree of potential spread points existed as the Victoria situation NSW would also go into lockdown. The only way States will be safe of this risk is for most of the population to be vaccinated.
