Hi Bernadette,
After you arrive go outside on a fine night - you will discover its hard to see much more than magnitude 3 stars, so it is essential to get out of the city to see much. There are a few groups that have star parties within reasonable reach of the CBD, considerably closer than Mangrove Mountain Pony Club or Bretti.
1. Our very own IIS group of diehards tries to make the trek to Katoomba one night each month in the Blue Mountains
http://www.iceinspace.com.au/forum/s...d.php?t=124712
There should be one in late September while you are here. This is highly weather-dependent and - I should add - the weather has been bad for most of the past year. This is a reasonable dark sky site at an airstrip at 1000m elevation, about a 2 hr drive west from the CBD. Usually plenty of scopes to go round, and someone may have a spare seat if you post in that thread; its a tad hard to find in the dark unless you are comfortable with using GPS. Bring warm clothing - a decent jacket, beanie and gloves and a thermos with something hot - as it really does get cold up there at night. Thermals are optional.
2. Northern Sydney AS sometimes meets at Turramurra Golf Course (this is on the northern fringe of the city, about 1hr from the CBD). A small group but usually there's a 12" scope. A few bright clusters, nebulae or galaxies can be had but its not going to do them justice, you really have to travel further out.
http://nsas.org.au/observing/
3. Sutherland AS has open nights at its observatory at Como, on the southern outskirts of the city about 45 minutes drive from the CBD
http://www.sasi.net.au/index.php?opt...d=22&Itemid=35 Similar seeing prospects to Turramurra.
As you already know there is Sydney Observatory, you need to make a booking
http://www.sydneyobservatory.com.au/...g/night-visit/
This is more of historical interest only as you really won't see much of the night sky there, it is about the worst possible location you could pick for stargazing.
Alternatively if you want to go on your own and are blessed with a perfect day for sight-seeing in the Blue Mountains, I would strongly suggest take a pair of binoculars and head for Hargraves lookout at Mt Blackheath, which is on a plateau west of Blackheath, in the upper Blue Mountains. This is an excellent dark-sky site. There also is an area of astroturf at Mt Blackheath lookout nearby (actually a paraglider launch) which you can lie down on.
To give you some idea this is the view in late afternoon
http://www.dpreview.com/galleries/73...mountains-pano
Comfortably beats anything in Ireland, I suggest
~~~~~