"Adelaide is not the entire country."
Fair point but not the full story.
At the moment, the low rates of positive results throughout the country are very encouraging. There's still a pretty high number of tests being performed and on mostly people who are symptomatic. Given infection rates are likely to be higher among such a group when compared to the rest of the population, the low confirmed infection rates are a good sign.
As well, the substantial majority of recently confirmed cases have been able to be tracked to known cases, indicating very low rates of within-community transmission. Victoria and NSW seem to be most at risk from within-community transmission and Victoria seems to have the higher risk profile.
I'm confident these facts were key considerations behind the decisions to allow (most) protests to go ahead last week, to allow larger crowds into stadia, and to open up borders next month.
That said, if the Melbourne case who attended the protest last weekend produces a new set of case clusters, there's time to rescind some of those decisions. e.g., keep the Victorian border(s) closed. Likewise, if new clusters pop up elsewhere.