All the Brachychiton populneus I have seen in the region are subsp. populneus. CNM lists both, with approximately the same numbers. I used to think they were not common but there seem to be many small ones now.
On the Pinnacle NR at least, there were a handful 5-10 years ago, but in the last few years we're finding more and more. Some are now getting quite mature -see https://canberra.naturemapr.org/Sightings/4251502 for example.