Over the last few weeks I've often seen the Blue Mountains from places near my flat in Crows Nest, such as the Harbour Bridge and the Pacific Highway. They're at least 50km away (80km for some of the higher ridges) but can be clearly seen on a majority of cloudless days.
I recall this happening only occasionally back in the late 90s; most days there was too much haze to see any part of the Blue Mountains. I don't know what it is (old sooty cars being taken off the road? fewer home fireplaces as people renovate?) but particulate emissions in the Sydney basin seem to be way down.