In general a valid passport is enough to gain access to most countries for the purpose of holidaying. However, a visit to Australia, even one that is limited to a period of three months, does require a visa. The only exception to this rule is if you are from New Zealand.
The visa that holidaymakers will require is the Electronic Travel Authority (often shortened to ETA) tourist visa. The ETA visa has no physical form, instead it is an electronic record stored in a secure database. This does not only increase the efficiency of the visa issuing process but also reduces the risk of forgeries and decreases the chance that anyone will be able to cheat the already scrupulously controlled system. Do not be concerned that there is no passport stamp/label associated with the ETA visa, officials will be able to access all the documentation they need simply by using your passport details. It is both simple and secure.
The Australia ETA visa allows its holders to travel to Australia multiple times in one year with each visit lasting a maximum of three months. There is of course no minimum period of stay, however, people tend to spend longer in Australia than most other holiday destinations, perhaps due to both the length of the flight required to access the country and the sheer vastness of the place. With so much to see multiple trips in one year is not infeasible.
An ETA visa does not allow people to work in Australia, the sole purpose of your visit must be for a holiday. This restriction includes students, however, those aged 18 to 30 may apply for the working holiday visa, which does allow holders to gain paid work in order to fund their stay. While holidaying in Australia under an ETA visa, visitors are allowed to study in Australia but only for as long as the maximum period of stay permitted by their visa. Applications must be made outside of Australia and it is recommended that you do not book your tickets until your Australia ETA visa has been approved.