Re Egg-Ended boilers..
These were quite common in NSW coal mines prior to about 1890.
They make excellent water tanks in that they are made from wrought iron and have no internal stays or gusset.
There is (or was last time I looked ) one still inn use near Newcastle as a water tanks at a ready mix concrete plant.
They were very inefficient (but with plenty of refuse slack coal who cared ?) and simple to operate and repair.They also had high thermal reserve capacity. (ie plenty of boiling water stored ready to flash into steam )
Several photos showing them in use at Newcastle mines (eg Co Operative and Greta Collieries) can be found in the Hunter Photobank and Newcastle University collections.
As to the maker or original owner of the one near Mt Kembla ... that is anyone's guess... interesting to hear it has survived.