Do I need a visa to travel to the United States?
It depends on your country of region. Certain country such as Australia, Singapore, Japan are under the Visa Waiver Program (VWP), which means citizens from these country doesn't need a visa to enter US. However if your country of origin doesn't fall in VWP, you will need to obtain a visa to enter US
What about Australia?
If your question is whether you need a visa to travel to Australia, then similarly to United States, it depends on your citizenship as well as the duration and reason of visit. Citizens of US, Canada, member states of EU, Japan, South Korea, Taiwan, Malaysia, Singapore, Brunei, and Hon Kong although technically need visas even for short visits could apply for an electronic visas online for the stays of 3 months or less. The visitors from New Zealand could apply for a visa upon arrival. The visitors from the rest of the world have to apply for non-electronic visas and could not visit Australia before receiving them. The provided information might contain some inaccuracies or be outdated. So if you plan to travel to Australia, it is advisable to check the relevant information at the websites of Australian Department of Home Affairs https://www.homeaffairs.gov.au/ and of the Australian Embassy in your country.

If your question is that if you need a visa to visit United States as an Australian citizen, then you don't a visa.