You can obtain an ETA from most airlines that fly to Australia as well as through a number of travel agencies. You can follow the more traditional route of going through Australian Embassies, High Commissions or Consulates.
You can also obtain your ETA online (for a fee) through the Australian government's website .
How much does it cost?
Many airlines and travel agents issue ETAs for free, as an act of customer service. However, some airlines or travel agents may charge a fee for processing the ETA for you.
If you set up your ETA online via the website that the Australian government operates, you will be required to pay a fee of 20 Australian Dollars.
As a side note, setting up an ETA is very quick so try to save yourself the fees, as it is more common for the ETA to be set up without fees if you go through the airline or travel agent than not.
What details do you need to provide?
Your passport information, along with the planned dates of travel/ travel details are required to complete the ETA.

