And finally to get back to your original question, the longest road trains operating anywhere on Australian public roads are quads from three A-trailers and a B-trailer. These I think can take the form of ABA quads, BAA quads or, as pictures in this case, AAB quads, also called 2AB quads, so an A-trailer followed by a dollied A-trailer followed by a B-trailer with an A-trailer.
However not all states and territories allow the same combinations. Generally the sparser population a state, the longer the permitted combinations. Don't quote me on this, I think these long quads only operate in Western Australia and the Northern Territory or something, on the other end of the spectrum New South Wales and Victoria only allow normal road trains (and maybe B-triples?) and within large cities even go as far as only allowing B-doubles.