From the information board at the gun:-
The gun was seized in France on 8th August 1918 by the Australian 31st Battalion in the course of an advance that shattered the German army. Throughout the summer the gun had fired on the city of Amiens from a distance of over 20 kilometres. Amiens was an important allied centre of communications.
The barrel itself had originally been a naval gun, which was converted in 1917 for use on land by mounting it on a railway carriage. The picture of the German crew with Bruno (as they called the gun) shows the number of people required to operate it. Its huge shells could cause formidable damage. As with most large guns, the barrel have a very limited life before it required re-lining.
During the Second World War, the carriage of the Amiens gun was used to test naval guns at Port Wakefield in South Australia. The carriage was eventually scrapped and the barrel and roof are all that remain.
Alas, the specifications are illegible. But there are references to the calibre as being 28cm!