If you want to buy Bitcoin with cash (AUD) through an Australian exchange, you will need to provide ID in accordance with Anti-Laundering Laws. Although, if you want to completely go off the radar there are other (less popular) options. If you visit hedgewithcrypto they provide a guide on other methods available in Oz, but if you're talking about larger amounts in cash to buy BTC, then it would be difficult to find a willing seller. Good luck.