Katoomba, Mangrove Mountain, for tonight, are probably within an hour or two of you.
Or several camping sites are available too, but these are a little further afield.
Best place in Sydney bar none is the observing site that the Northern Sydney Astro Society uses at Terrey Hills - and it should be awesome tonight. Tonight is members-only night but if you choose to go I doubt they'd mind if you signed up. There should be many decent scopes and on a good night the magellanic clouds are easily seen naked eye. Moon a few days old will be a fine sight as it sets at dusk so get there early, say 5:30 at the latest. Details on their website.
Observing at Katoomba is no more as the fellow that ran the property has passed away. If you can do the drive, the best dark locations are Hargraves Lookout or Mt Blackheath, both on Shipley Plateau west of Blackheath. Further than that means an all-weekend trip.
Other sites publicly accessible are Table Rock at Kings Tableland, south of Wentworth Falls (OK but not terribly dark as the light dome of Sydney is still bright there), the Mt Banks carpark, off Bells Line of Rd west of Mt Tomah; a disused airstrip at Clarence on Bells Line of Rd before it descends into Lithgow.
There's also the ASNSW observatory at Mt Bowen, or the one at Linden. Both should be functioning.