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