An ETA visa is an electronic travel authorization, which is often offered to citizens whose nations are exempt from applying for visa prior to arrival. Several countries offer ETA or their own variant of ETA from Australia, Canada, to Singapore, but the modalities may differ. For example with the Australian ETA, you would be well advised to apply in advance since the approval process can take a while. Whereas in the case of Singapore, it is more or less immediate, and moreover, nations often enter into reciprocal agreement so that if one nation was to grant the other with ETA status, then the later would have to reply in kind. This is why; you may find such ETA agreements in place between Canada and Singapore, Australia and United States and more.
Australian ETA, what it means:
The Australian ETA is one that you can apply at any point of time as long as you happen to be a citizen of one of these countries (Brunei, South Korea, Hong Kong, United States and more). If you happen to belong to one of these nations, then you can apply for your electronic travel authorization at any point of time. But when applying for an AU VISA, it would be in your best interest to apply well ahead, since the approval process is not immediate and can take a while. Please note, you need to have your ETA approved, before you can use the same to enter the nation legally.
Please ensure that all the information contained on your passport and your online application happens to be the same. Your ETA would be electronically linked together and it is important that they sync well – that is, they contain the same details. In case of any discrepancy, your application would most likely be rejected so double check the same, before sending the application over.