Traveling with Two Passports to Australia

Sorry if you're seeing this again. Apparently my post disappeared in the site crash... not to worry, here's the jist of it...

I currently hold both US and UK passports. I know that in general, the rule is to always leave and enter your country on the same passport. I know that it's a felony for a US citizen to enter the country on a foreign passport (yikes!), so whenever I enter/leave America, I always do so on my US passport... And I do pretty much the same thing at Heathrow, except I show the check-in assistant my US passport (if I'm not doing self check-in). Usually I've never had to worry about which passport I choose when I've flown between the US and the UK. Even if my point of entry in the EU is another member state, such as Ireland or Spain, I'll just show my EU passport... But here's my question (get on with it, zootvd!)...

I'm planning to travel to SYD from LAX and back again (currently living in Los Angeles). Would there be a problem if I wanted to use my UK passport upon entry @ Kingsford Smith, upon my departure from Australia, and then just use my US passport upon my arrival at LAX?

I've personally never been to Oz (I'm the only one in my family that hasn't been Down Under), but I know that both US and UK nationals have to get an Electronic Travel Authority visa online...

I know that to make things very simple, I should just stick with my US passport, but I prefer traveling on my UK one. Any thoughts, ideas or answers would be much appreciated! Thanks!

OK ... you will need the Electronic Travel Authority (ETA) in which ever one you enter Australia. I'm not 100% sure but I think the carrier checks if you have the ETA ? (otherwise they foot the bill when you get turned back).... . So if you if you put the ETA on your UK passport and then check-in with US passport which has no ETA, they may not let you travel .... hope that makes sense and Im sure someone will correct me if I'm wrong [:)] (I'm also puzzled why you need to do this.. both passports enter Australia with equal ease [?])

Cheers
Mark[:D]

If there's no point in doing it, then I'll just stick with my US passport. I just wanted to see if there was any advantage in using my UK passport since Australia is a part of the Commonwealth of Nations...