I can't give a definitive answer for NSWGR but think it was similar to Victorian Railways. Bullhead rail was never used in Victoria, railways built up to 1863 (basically the main lines to Bendigo and Ballarat) were built with double headed rails in chairs and all the government lines after the 1860s was built with flat bottom rails and no chairs. The chaired track remained in use on the main lines until the early 20th Century (with replacement 80lb steel double headed rail) and was then progressively replaced with 80 or 100lb flat bottom rail. As a government owned railway capital expenditure was always harder to obtain so anything serviceable would be reused in sidings or short branches to army bases etc. There is still a short length of inside keyed chaired track in daily use in the Ballarat carriage shed/sidings.
NSW was similar, the early lines were double headed rail in chairs and I think by the 1870s they also adopted flat bottom rail. In the relatively dry climate in Australia steel rail remains serviceable on lightly used lines for well over 100 years so any early chaired track relaid in sidings could have lasted into the 1980s as it did in Victoria. The other thing that struck me looking through a book of early NSWGR photos was they seem to have ballasted to near the head of the rails in yards for most of the 19th Century, longer than elsewhere, so it is difficult to tell if the track is chaired or flat bottom and no need to model sleepers.
