Clarifying Sydney; the 400 bus has been withdrawn and replaced by separate services - 350 and 420.
The 350 runs from Domestic to Bondi Junction (along a different route to the old 400).
The 420 runs from Mascot station to Burwood via Domestic and International. It also passes Rockdale (handy for interchange to and from Wollongong).
Normal Opal fares apply on each service. Seniors are well advised to get an Opal card because the $2.50 maximum fare (for one day) applies. It's one of Australia's best travel bargains. As one would expect the "gate pass" at the airport is always added regardless of the type of Opal.
During a recent visit to Sydney I confirmed there is a "special" train fare between Domestic and International that is a substantial saving on payint the gate pass twice. However, it is still more expensive than using Opal on the 420.
There is also the free "T-bus" between Domestic and International terminals.