Unless you are a New Zealand citizen granted visa exemption on arrival (in most cases), you will need to obtain a visa prior to visiting Australia. Citizens of all European Union countries, as well as Andorra, Norway, Iceland, San Marino, Switzerland, Liechtenstein, Monaco, and Vatican City are eligible for an eVisitor visa, which can be applied for online. For citizens of the United States, Canada, Japan, Hong Kong, Singapore, Brunei, South Korea and Malaysia, the Electronic Travel Authority (ETA) entry permit is available, which can be applied for online and grants holders the right to enter Australia an ulimited amount of times for stays of no longer than 3 months per visit.
Citizens of all other world countries (with the exception of Somalia) are also able to apply for a visitor visa online. For more information, visit the official website of the Australian government's department of home affairs: https://www.homeaffairs.gov.au/lega/lega/form/immi-faqs/what-visa-do-i-need-to-visit-australia