I remember reading that there was no reason that Australia wasn't a rainforest type enviroment except that b/c abos set fire to the countryside as a hunting technique to run game out of bush so they may hunt it.
They have definitely had a major effect on the ecology of Australia. Starting with the extinction of almost all land animals >100lb about 45000 years ago (https://phys.org/news/2017-01-humans-climate-australian-megafauna.html).
The continent wouldn't likely be covered in rainforest though, as the geography tends to leave the interior of the country dry. If there was a substantial mountain range on the Western side of the continent, rather than a smallish mountain range on the East, then things might be different (similar to South America). But, yes, the fire based hunting practices of abos over a long time scale would have a significant effect on the flora and fauna that are able to survive such an environment.
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