Land law in Indonesia is complex.
Land law in Indonesia involving foreigners is especially so.
You may do best to contact a law firm in Indonesia, probably in the province where he lived.
A google search will be helpful.
If you have no real sense what you are looking for, then start by looking for a lawyer in your state in Australia who speaks Bahasa Indonesia ("Indonesian"), and see if they can refer you on.
I'd make suggestions, but the Community Rules don't allow us to point to particular firms.