It is entirely feasible that Australia would support human life in your scenario.
In the experiments in Biosphere 2
six people were capable of being sustained for many months or years in a sealed environment of 1.27 hectares. Although there were many issues connected with the absorption of CO2 by the concrete, species die back and personal conflicts within the team it is reasonable to deduce from these experiments that these could have been resolved in time. And in your scenario many of the issues would not be present (presumably). No excessive proportions of concrete for example.
And this appears even more likely in a larger biosphere. If the biosphere had been 12.7 hectares rather than 1.27 and the concrete had been sealed it is extremely likely that the team inside could have survived indefinitely.
In your example of Australia it would seem even more likely still as there are something like 46 million hectares of arable land available in Australia
So to directly answer your question there is definitely enough vegetation in Australia to support human life. The real question is how many animals/humans can be supported? This would depend on how many of each sort and many other complex factors. But based on the above it would seem that 42 million humans should be supportable provided the animal population was carefully controlled.