Flying a flag upside down is a symbol that has been used by many countries to signify conditions of dire distress or extreme danger to life or property.

At the risk of cultural appropriation, the symbolism of an upside-down aboriginal flag reflecting the smoke red/orange sky and blackened ground seen throughout much of Australia right now seems to be a deeply fitting expression of the distress our land, its people, wildlife and cultures are experiencing.

The Australian Aboriginal Flag represents Aboriginal Australians. It is one of the officially proclaimed flags of Australia, and holds special legal and political status. It is often flown together with the national flag and with the Torres Strait Islander Flag, which is also an official proclaimed flag.
The Australian Aboriginal Flag was designed in 1971 by Aboriginal artist Harold Thomas, who is descended from the Luritja people of Central Australia and holds intellectual property rights to the flag’s design. The flag was originally designed for the land rights movement, and it became a symbol of the Aboriginal people of Australia.

Wikipedia.