I've scoured everything from Wollongong to Newcastle and as far inland as Bathurst. Basically the best sites are the upper blue mountains.
South of Sydney is no good - there are no really good sites; the few I found are prone to fog and light pollution from nearby towns. In case you are tempted, Bald Hill above Stanwell Park (I fly there sometimes) is hopeless at night as the wind off the sea will quickly leave a greasy, sticky, salty and corrosive film on your optics. The only coastal sites I found any good were Saddleback (a big hill southwest of Kiama, great view) and Cambewarra Mountain which is well inland. There are a few sites on the backroads between Bowral and Kiama, especially around Robertson and Jamberoo, but that's a long way for a nights stargazing and the seeing is nothing special.
Southwest (along the M5) takes you to the cloudiest part of Australia (southern highlands)... forget it. There is a dirt airstrip at Bargo which SAS were using, but it is very dusty and everything will come home coated in fine red bulldust.
North... Past Berowra there's only the F3, I have tried observing at a house at Mt White, it was fair. But to get dark skies you'll have to go a lot further and inland away from the built-up areas in the Hunter Valley.
So that leaves west.
1. Kings Tableland (before Wentworth Falls) - turn into Hordern Rd, drive to the western end then left (south) down a dirt road that follows the ridge. A few yards along you'll find a locked gate (horizontal pole thing) and a really nice big slab of sandstone on the ridge, it faces west.
2. Leura oval - it is dark at night (no lights) but the dew is shocking as this is black soil and manicured.
3. by prior arrangement with the manager, the Katoomba Airstrip; you'll find other IIS members going there sometimes. Depending on where you live it may be an easy drive (for me it's 1h 45). If you are early you'll score a spot on a concrete pad
It's fine for casual observing and some beginners starting to take photos but it isn't as dark as the next sites... Good compromise if you're going home about midnight.
Note: Keep an eye in the Star Parties section on IIS for the next time its planned.
4. Hargraves Lookout, at the end of Shipley Plateau Rd, via Blackheath. Very dark and turns on great seeing as the airflow is coming across a huge valley and you're up high, so if its fairly quiet the air will be laminar, ie VERY good seeing. Very low horizons E/W and south, but also very exposed (and cold) if it is windy. You will need to be well dressed for this site.
NOTE: Both Katoomba Airstrip and Hargraves are on pale sandy ground and sandstone so dew is generally not a problem.
It also suits me as I go paragliding by day from Mt Blackheath which is quite close by.
5. carpark at Mt Banks, on Bells Line of Rd, similar to the Katoomba Airstrip.
6. The picnic area and also Cathedral camping ground at Mt Wilson, though both are problematic and not good on your own as there can be big dogs roaming at night and in the dark they are a serious problem. If you're in a group it would be OK.
7. Lithgow Scenic Airways airstrip, lefthand side of Bells Line of Rd before the descent to Lithgow. No problems. Very dark and great seeing.
8. Hassan's Walls, out of Lithgow is a superb site on a calm night, but dew can be heavy. 2.5 hour drive.
9. If you want a place for a weekend with the family AND some observing I can recommend the Woolshed cabins or The Potters Cottage, both in Centennial Glen.
10. For the budget conscious (and brave) there is a little secret place for camping too - Blackheath has a camping ground in quite a nice setting.
11. Places to eat - the Ivanhoe in Blackheath does great counter meals (and quick too), for the price of a pizza (the Olive Tree at Wentworth Falls is OK).