As a general reference, use ‘Aboriginal and Torres Strait Islander peoples’. Most communities accept this as an appropriate term. Note that ‘peoples’ is plural when you're referring to a group of communities, but it should be singular for one group of individuals.
Avoid using the acronym ‘ATSI’, as it can sound dehumanising and disrespectful. For a shortened form, use ‘Indigenous’ with an initial capital. But be aware that the word is contentious for some because it fails to recognise the diversity of the communities involved.
The phrase ‘First Nations’ is also gaining traction around the world, as an adjective to describe the first peoples of a country that was later colonised.
Whichever term you use, avoid pairing it with ‘Australians’ unless you need to refer to different indigenous populations around the world.
If you need to speak about the non-Indigenous population, opt for that phrase rather than terms like ‘white’ or ‘Caucasian’, which are highly problematic.
