Chinatown has an abundance of authentic Yum Cha joints, but one of the standouts is Golden Harbour Restaurant. It has a nice atmosphere, the dishes are all tasty and well-portioned, and the staff don't mind giving you an explanation of each one.
The best Yum Cha restaurants in Sydney aren't (only) in Chinatown anymore. Try going to places where there is a large Chinese population like Eastwood, Carlingford or Cabramatta. There is also a fab Yum Cha in the Ikea complex at Rhodes (Phoenix). But my favourite place is Eastwood (district pictured)... lot's of choices for great Chinese and Korean food there, Yum Cha and otherwise!
Love Marigold in Citymark. Love those prawn dumplings, and they have some other cool treats such as Mango Pancakes. Din Tai Fung at World Square is also great. There was a Yum Cha train in Westfield Sydney that was excellent and great prices. Unfortunately they're now closed.