OK, a quick review of situation from International Terminal. As I had time up my sleeve after a morning arrival, into Sydney I took the 400 bus and planned on changing to train at Rockdale. However the bus also goes past Banksia station and I decided to change there, as there seemed to be a reasonable number of people waiting for train (during morning peak), and I only waited about 3 mins for train to city.
Coming back to airport yesterday, I planned via sydney transport website, and got on the Waterfall train at Town Hall, again changing at Banksia where it was a 8 min wait for the 400 bus (3 mins at Rockdale for the change seemed pushing it). Total travel time from Town Hall to Airport was 35 mins - if I had taken the airport train from Town Hall it would have been 25 mins as it goes via Circular Quay (maybe 2-3 mins less if I went to Central first and changed). It is a little bit of bother doing it this way though, and although I saved the cost of a weekly gate pass, for me it's more the principle rather than the dollars it saved.