This project was announced in 2015, and still hasn’t opened – but it’s probably complete. This is what I said in 2017:
Finally found out where the 1880 dome viewing platform of the #RoyalExhibitionBuilding was located. Plans are progressing to make it accessible again, with a lift escape stair etc hidden within the south transept corner tower (the other side). Capacity rules will limit numbers to 150 at a time – and they’re expecting it to be so popular there’ll be a bit of queuing (entry via the basement on the south side, plenty of room for ‘immersive interpretation’ while-u-wait ?
Update : found a lot of construction photos from 2019 at @lovellchen