Upvote:9
ALL Australian citizens can use Smartgates at Australian airports. These are, by far, the fastest means of entering Australia.
US Citizens can also use these gates, however this isn't relevant to you as to enter using a US passport you would require a Visa/eTA which you won't have - plus you are supposed to use your Australian passport if you are an Australian citizen.
You seem to be referring to a previous provision that allowed some US citizens, such as those with Global Entry, to use the SmartGates when other passengers could not. This program has not existed for several years since they started allowing ALL US Citizens (as well as citizens from many other countries) to use the SmartGates. ie, there is no 'USA classified fast lane' (and really there never was - at least from the perspective of someone with an Australian passport)
And yes, I am 100% sure of what happens at Australian Airports.