Ultimate Hobbies in Parramatta is very good. Immaculate shop, great guys, they sold me my T-Rex 500. Not heli-specific but their heli expert is a good guy and loves to chat. Parramatta is a little far-flung from the CBD (half-hour by train) but definitely doable.
Avoid HobbyCo in the CBD. Apparently they used to be acceptable, then started hiring McDonald's fryroom dropouts and 14yo girls who want to own a pony. No range, inconsistent advice, pretty much a nightmare all-round.
I hope you have a great time! May I recommend the Zoo if you're there with your family. Truly a beautiful place.