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.
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.