If you're visiting Australia for no more than three months, traveling with an airline, and you are a citizen of the United States, United Kingdom, Canada or of certain other countries, you may not need an Australian visa as such but may need an electronic travel authority (ETA) in lieu.
For visitors to Australia, a three-month stay is often very much a top limit, so for citizens of certain designated countries, all you probably need is an ETA.
To apply for and get an electronic travel authority, visit eta.immi.gov.au.
Update: From October 27, 2008, eligible passport holders from the European Union and other European ETA eligible countries should apply for an eVisitor instead of an ETA. The eVisitor is for travelers seeking to visit Australia for business or tourism purposes for up to three months.
The times you would need an Australian visa (instead of an ETA) to travel to Sydney and other parts of Australia is when you are traveling on a cruise ship, you want to stay in Australia for more than three months, you hold a passport of a country not eligible for an ETA, or if you plan to stay permanently.
If you were thinking of becoming an Australian resident, see what's needed at the Department of Immigration site.
- Note: Australian entry requirements are subject to change and intending visitors should check with official Australian government offices for the latest information.