I tried two more second hand bookstores yesterday. Books Buy and Sell no longer exists, it has been replaced with a sex toy store. I guess that says something about Sydney again. Gould’s books was all I had remembered (big mounds of books everywhere), and was worth the visit. Its more organised than I remember, and I did eventually find the science fiction section. Thanks to those who recommended it in comments here. It is surprising that a second hand book store managed to have no paperback Isaac Asimov, with the four Harry Harrisons I bought make up for that I suppose.
I took some photos of Gould’s as well, but it turns out that the room full of PhDs can’t figure out how to empty a micro SD card, so I will give up on putting them online for now.