Absolutely, opening another persons mail is illegal in Australia.
The only people allowed to open another persons mail is customs and border control if, and only if the mail is deemed suspicious.
Throwing away the old tenants mail in your new rental is also illegal. Only Australia Post staff are allowed to do that once the necessary actions to find the intended recipient have been attempted.
Opening someone else’s mail is illegal in;
- New South Wales
- Australian Capital Territory
- South Australia
- Western Australia
- Northern Territory