Been there a lot over the years, and my partner was a caretaker there for quite a few years.
Local lighting is not an issue, the skies are OK to pretty good but not truly dark. It's just too close to Sydney for that. There's interesting heritage stuff to see there, some good walks, old mine sites to explore, etc. You will drive through areas with darker skies than Yerranderie whether you go via Oberon or Goulburn, so I wouldn't go there for dark skies, but if you want to mix a bit of observing with having a look around, then by all means!
Get in quick before its locked up, I say.
Al.