Hi Sandy1000, it looks to me like it could be a 5" BL Gun on MKI Vavasseur carraige, I think these were intended as shore mounts from around the 1890s or so.... probarbly a shore mounted conversion of an older naval gun....
These guns look like the Cairns coastal defence gun in the thread "What and where is this gun". The 6" Cairns gun was installed in 1887 so the dates of the Cerberus guns are about right. I wonder if they came from one of the trio of forts around the entrance to Port Phillip Bay (Point Nepean, Swan Island, Queenscliff) - I think Nepean had disappearing guns in the 1890s.