Also, do not assume you will be approved automatically for your ETA. I was recently denied an ETA and I had to go to the Australian Embassy for a quick interview. Here are the facts:

1) Bought ticket cash for same day departure to Australia from Indonesia on Garuda.

2) Travelling on British Passport

3) Denied ETA and Garuda refused to carry me. Because it was Indonesian election day, the Australian embassy was closed - had to delay my trip for 48 hours.

In the end it was no problem but I was worried for a sec as I am actually Australian but I was travelling on British Passport as my Australian Passport was with another Embassy getting a Visa processed.

Do not take it for granted.