why did the Australian megafauna die out after human arrival?

eaten or hunted - they were either regarded as a food source and eaten or as a threat and hunted.

compare Australia to Africa......

Australia is a small isolated island continent and for thousands of years remained so. relatively speaking the area of land capable of supporting larger animals in Australia is quite small.... this allows a particular type of self contained Eco system to develop in even further isolated pockets on Australia.

Africa on the other hand is connected to the largest land mass on the planet Eurasia and has a very different less "fragile" Eco system

Australia's isolation plays a large part in all of this.... when the original settles turned up in Australia they caused a massive imbalance in the food chain.... suddenly the previously untouchable mega fauna became anothers prey and so were quickly wiped out.... whereas in Africa/Eurasia there is far more species of mega fauna for humans to consume over a much greater area

also when white European settlers came they brought their own species with them.... case in point rabbits... this further upset the balance and more animals went extinct

in short... Australia's isolation and relatively small size facilated the demise of the mega fauna due to human consumption