There are currently no travel warnings issued by the U.S., E.U., or South Africa that advise against travel to Australia. The most recent Australian travel warnings issued by these countries were issued in October of 2002 after nightclub bombings in Bali killed a number of Australian tourists. However, this is subject to change. Please check with your government before traveling to Australia.
Australia has a four-level system of counter-terrorism alerts, which includes low, medium, high and extreme level distinctions. It changed to a four-tiered system in June of 2003, replacing the three-level system that had been in place since 1978. The current threat level is medium, meaning there is a medium risk of a terrorist attack in Australia. This level has been in place since the September 2001 attacks on the United States. For more information, check out the Australian National Security webpage.