Yeah that is a possibility. The extension of this lockdown was based on modelling that suggested numbers needed to be reduced further to prevent that scenario. I am glad they are using evidence based decision making, although I wish they were more transparent with their model, as some academic epidemiologists have questioned it as being too pessimistic.
Australia does have good testing infrastructure. The hotel quarantine failed last time, hopefully that has been addressed. Contact tracing stands a better chance with low case numbers than high numbers. My friend is a nurse seconded to contact tracing in Melbourne. She said during the peak (700-cases/day) they were so pressured for time they could only go back 2 days per person. Whereas now they are able to go back 2 weeks. Anecdotal obviously, but does not fill me with faith that our contact tracing system can cope with large numbers.
So yes, in an ideal world you would be able to rely on mask wearing, social distancing, not going to work when sick, test, trace, and isolate to bring numbers down. But that is a lot more likely to work if you have the virus under control to begin with and lockdowns are a tool to do that. This is what is currently happening (minus compulsory masks) in New South Wales.