It's certainly not impossible, but I think there are a coupe of issues. One is that part of its potential footprint is occupied by the Seacliff bridge. A pity they couldn't have thought ahead and designed a road-rail bridge. The other issue is that, whereas that Hong Kong bridge seems to be in a fairly placid marine environment, the south coast gets smashed by the Tasman Sea which, in full force, is very powerful, leading to potential damage and service interruption. Either way, it's all subject to geological studies.
It seems to me that the drift of things here is the the Waterfall-Thirroul section of the line will be a difficult one to solve. There will doubtless be a solution but it will be expensive and government has so far shied away from tackling it. The best path for discussion in the shorter term is to look at performance along the potentially fast sections of the line (which actually constitute about 80% of its length) between Sydney and Waterfall and between Thirroul and Bomaderry. These sections, which contain nothing serious in curvature or gradient that would inhibit the performance of electric trains, are presently run waaaay too slowly. How to fix that?