I have a valid ESTA, do I still need a visa for the USA?

No, if you have a valid ESTA authorisation and qualify for the Visa Waiver Program, you don't need a separate visa for stays of up to 90 days for tourism or business.

Any advice is general advice only and does not take into account your individual objectives, financial situation, or needs. As entry and visa requirements for the US may change over time, it is your responsibility to verify entry and visa requirements with the official government authorities including the US Department of State - Bureau of Consular Affairs, the United States Customs and Border Protection (CBP), the US Embassy in Australia or your local US Consulate.