Our first trip to Australia was 2001. I could not remember whether it required ETA.
We did several trips to Australia between the years of 2001 to 2009. Then the AUD went up too much we stopped going down under.
I only had emails related to trips from 2009 onward. Just looked back, ETA already existed in 2009. Of course it was required on each subsequent trip to Australia since then.
One thing I must say, the Australia Immigration / Custom offices are very efficient. In 2016 we entered Australia with HKSAR passports, the ETAs were attached to the HKSAR passports. At leaving we accidentally used our US passports. I was afraid Australia records would show we overstayed in Australia due to the HKSAR passports were not used on exit. Took me a lot of time to finally connect to the right departments to check our records in Australia Immigration system - Well, it showed us left Australia on the exact date we flew out, despite we used different passports. I want to say, the Australia immigration offices are very patient and helpful regarding my enquiries.
FYI, if you overstay not only you would be subj to a fine / sometimes prison term, but you would be forbidden to enter Australia for 3 years, from what I researched. Hence I was determined to get the records straight.
We again entered Australia in 2019, no issue getting the ETAs.
