Owen,
I am extremely jealous, I have never had the opportunity to visit the workshops. Any info on who to contact and when to get this access? PM me if you like. I am based near Brisbane but visit Canberra fairly regularly, I always have a look in the War Memorial.
As to the gun, the cipher on top will tell us a lot, not very clear in the above photos. I agree on the size being about that of a six pounder. It appears by the marks and damage it must have been a display gun somewhere indoors. Not enough damage and the barrel isn't blocked so that rules out a public park. I have several reference books on Australian Coastal Defence guns, more details and I reckon I could lock down its identity.
Shooter2