Fully agree, though I didn't know the exact numbersIf agriculture is invented in Australia about as early as other centres (say, 8000 BCE), there's no need for colonisation. Any plausible rate of population growth is going to see every last part of the farmable areas of the continent filled up to carrying capacity, long before any plausible colonisers [1] appear.
To put a few very generous numbers on it, assume that the carrying capacity of Australia, with the new crops, is 100 million. That's probably much higher than is realistic, but it's a good round ballpark. Let's also assume that the population growth of these early agriculturalists is a very low 0.1% per year. That's probably also a very generous under-estimate, but it will do. For further fun, suppose that there's also only 1000 of these agriculturalists. That's also far below any reasonable population figure for hunter-gatherer Australia, but assume that the remaining peoples refuse to take up agriculture, and there's no significant interbreeding.
Have a guess in what year Australia will reach its carrying capacity?
Answer: 6842 BCE.
Population growth is a wonderful thing.
[1] Excepting New Guinean colonisers, perhaps.
New Guinea was very early in its agricultural development (as you probably already knew), so it could be that a good jumpstart from Kuk Swamp is all you would need for the Queensland Tropical Rainforests, and maybe even the rest of the northern third of the continent, like the Top End and the Kimberly Regions: Much as with maize in the OTL Eastern Agricultural Complex, New Guinean crops such as taro, yams, bananas, and (complete!) protein-rich winged beans could eventually complement indigenous crops such as wattles, boabs (native baobabs), pencil yams, some native species she-oak (used in New Guinea as a nitrogen-fixing crop), along with native and New Guinean livestock such as native fish (like eel-tailed catfish, jade perch, yabbies, redclaws, prawns, flood plain mussels, Murray cod and barramundi) sugarbag bees for pollenation and sugarbag, goanna, dogs as well as pigs from New Guinea. Eventually, other crops could come in through Indonesia and the Polynesians.