The premise of the article seems to be that high density cities are less energy efficient and therefore not sustainable. I think that might be debatable in itself. But in any case, that's only true if you use non sustainable sources of energy. Australia particularly has no excuse to not power it self using just wind and solar (+ batteries). They'll probably get there eventually. Switch to clean energy and then sustainability has to be about other things than energy usage.

The biggest problem Australia has is climate (heat waves) and water shortages. Even that is a solvable problem given that they are surrounded by oceans that you can produce clean water from with sustainable energy. Sustainable farming is a solution here as well.

The counter argument argument to this article is that spreading millions of people all over the place would be hugely disruptive to nature. Wherever we go, we chop down trees, turn forests into farmland, and quite often into dessert eventually. We cut roads through nature, pollute our rivers, etc. Not sustainable at all. There are soon going to be 10 billion people on this planet. The only way to sustain that is to keep most of those in cities.