you may consider Tasmania as opposed to gc given that you have 1.5 months. it will take you 8-10 there, I will consider to do Tasmania because it will save you another connecting flights as you will be in sydney already. then do another trip separately going to Queensland where you can do cairns and gc because there're direct flights from HK.
In fact, there are a variety of things to do around sydney. I highly recommend Port Stephens, it offers things very similar to those in gold coast, except for amusement parks. it's tranquil, not as many ppl as in gc.
then can drive to hunter valley for food and wine, blue mountain. there is a decent size of zoo situated south of sydney where you can cuddle koalas and wombats, and mostly locals go there.
Last time, I was in Sydney for 14 days as well. I go to Woloogoon, Kiama, Blue Mountain, Hunter Valley and Port Stephens. I missed The Entrance. You must visit there.
In fact, 14 days for Sydney is very good. You can go to many places. Just Sydney near the City, you can go to the Zoo, go to weekend market, Visit Opera House, relax at Darling Harbout, try the Chinese Seafood. Sooooo many things you can do. If you can drive on your own is even better!