Bush Tucker is the answer.

Bush Tucker is a colloquial term for any plant, insect, seed, meat etc that can be sourced from wild sources. Indigenous Australians were largely Hunter Gatherers though we do have evidence of agricultural endeavours such as the cultivation of crops (tubers and root vegetables) and in South-West Victoria we have the preserved site of 7000 year old eel farms. This really is an incredible site. The Budj Bim eel farm is located in a lake formed in the crater of an extinct volcano. The traditional owners, the Gunditjmara people, carved channels out of the solidified lava flows to channel eels into pools. As the water level rose and fell, the eels would become trapped, allowing the people living in the area to harvest them. They also used separate pools to hold live eels to keep them fresh. The lake was fed by a water source they never ran dry and there is evidence of permanent indigenous settlement due to the reliability of the eels as a food sources.

Naturally, food sources and diets varied with different regions, Indigenous people living in Central Australia sourced meat from lizards and snakes along with kangaroos and other native mammals whilst also collecting berries and fruits from furit bearing plants. Insects were also a major source of protein including the truly horrible looking Witchetty Grub which I have tasted and was personally not a fan of. Honey and other nectars were also collected, providing a sweet alternative to diets and seeds could be ground and made into a dough which in turn could be baked into a seedcake.

Indigenous people hailing from the more coastal regions unsurprisingly sourced shellfish, sharks, turtles, fish and sea cucumber (trepang). The sea cucumbers in particular are interesting as they were (and still are) considered a delicacy by many South East Asian cultures. Prior to European settlement, sea cucumber were traded with Makassan (an Indonesian people) traders who would sail south to engage the indigenous peoples of Arnhem Land in trade.

Along with all of the above, there was also widespread hunting of marsupials that you already alluded to, kangaroos, wombats, echidna, possums etc as well as emus and other birds.

This is an incredibly common question. I answered a similar question here, although the focus was more on why native Australian foods are not commonly eaten today. It both answers your question and maybe gives a little more context.