South east Asia via land bridges during ice ages is the current dominant theory I believe. Indonesia wouldn’t be accurate, more like Papua New Guinea - but both of those countries are far newer than when the original Aboriginal’s descendants crossed. Genetically they’re closest to PNG AFAIK.

As an aside, Dingoes (genetically distinct from dogs, Canus familiaris) came over much more recently, also believed to be during a land bridge during an ice age ~3,500 years ago from south east Asia - specifically PNG also. They’re most closely related to New Guinean Singing Dogs and share the same taxonomic rank under ‘Canus lupus dingo’.