There is no such place accessible by coach within 2 hours of Sydney. After 35 years of chasing every dirt road this side of Bowral, Bathurst to the Hunter, if there was such a place stargazers would be all over it, we'd all love a big tar road to a big tar car park in a dark place.
Anything that big will stand out on Google earth and quite simply there isn't one in the national park, apart from:
Euroka, in the national park on the south side of Glenbrook, good luck getting there in any large vehicle. Not very dark.
Govetts leap, and that's hopeless (street lights and fog is frequent).
Mount wilson - cathedral camping ground may be accessible for you.
Lake Lyall near Lithgow.
Rydal, a tiny hamlet on the train line near Lithgow; has a showground with excellent sky, and there's a conference centre of some sort that would love some business.
Lake st Clair camping ground north of singleton is the only place that might be suitable and that means an overnight stay. Pretty good skies.
You could visit all these for a look over 2 clear nights.
Alternatively:
1. Get in touch with local landowners (ie farmers) asking if they'd accommodate this for a fee. Anywhere west of the range from Hartley, and try to get on a hilltop as the valleys fill with fog most nights. When Halley's Comet was visible in 85-6 this is what the Japanese did - a jumbo plane load per day were ferried out to a farm near Bathurst.
2. go to Echo Point, and ask the BMCC if they'd turn all the lights out at Katoomba for an hour when you want. Fat chance, I'd say.
If you do get this off the ground I'd suggest doing what a couple of resorts do - take a knowledgeable astro guide, one or two decent telescopes and several pairs of binoculars for people to get a good look. Especially if the planets are up, bright galaxies or a comet - you'll get requests very quickly.
By way of example this is one of the attractions at Mt Cook, in NZ where they do exactly this.
