That's the Powerhouse Museum, located between Pyrmont and Ultimo. If you could hover above it in a helicopter, the view would look something like this:
Imagine if you could magically click a link and jump back in time, and see the same view from sometime between 1900 and 1939...
If you're lost: in the older picture, look beyond the chimneystack, and just to the left. Immediately beyond the chimney is a vacant block of land; at the top end of this is a short road, which has on the right a not-quite-right-angle corner. The road then continues up the picture - but it's not quite straight, it bends a little to the right. If you look at the modern picture, you can see the same not-quite-right-angle corner and the same not-quite-straight road - although now the Western Distributor flys across the not-quite-straight road.
See the large building inside the block bounded by the not-quite-straight road? That building is Global Switch Sydney - built in the last days of "Build it and they will come". It's only in the last few years that it's starting to reach full capacity.