The verbal estimate I have is my current ceiling insulation should have an R value of 3.5, but with its current state it's probably 2.5ish. In #Canberra, any ceiling with an R value of less than R must be upgraded to at least R5 to be rented to a tenant: https://www.climatechoices.act.gov.au/policy-programs/sustainable-household-scheme/buyers-guides/making-homes-healthy-all-year-a-guide-to-ceiling-insulation
I'm in NSW, so I'm not legally subject to a rule like that (yet!), but that seems like a good target to aim for, especially since I also live here.