We traveled to Australia with HKSAR with ETAs as usual, but on the departure due to we were flying back to US we checked in at QF F lounge at MEL with our US passports. After check in there was a ground staff escorted us to the security check point where we were guided to the special lane and went thru the security check quickly. All the while we had our US passports in hand, totally forgot to switch to the HKSAR passports for exit purpose. We did not expect the immigration was e-Gates and just a few steps after security. We went thru the e-Gates with our US passports, not a single moment I thought about it - UNTIL much later when we had settled in at the QF lounge when I started to putting our docs away then I realized we should have used the HKSAR because it is the doc the ETA attached.

So now in Australia immigration records, it may seem we have never left Australia?

Would we be deny entry when we revisit Australia under the same ETA within its validity?

I kept our QF boarding passes, the slips of the Global Entry at LAX, as well as the BPs on the subsequent flight on AS as proofs of our departure from Australia, that we have not overstayed the 3 months allowance.

Our US Passports expire Mar 2019 so we would have new US passports. Should we reapply ETA with our new US passports? The names on both nationality are identical, and of course the DOBs.