These 9 inch Rifled Muzzle Loading guns are 12 ton Armstrong Fortress guns (300 pounder guns). The pair of them are situated in Williamstown (near my parents' house), an inner suburb of Melbourne, and cover the final approaches to the port. This pair (Nos 1679 and 1683) were initially emplaced at Fort Gellibrand less than a mile away to cover the southern approaches to the harbour. The were resited when the Fort was provided updated models a few years later, as shown here on the surviving ramparts:
From the Military History and Heritage of Victoria website here : http://www.mhhv.org.au/?p=2351
The Point Gellibrand shore batteries were first developed as part of an immediate defensive system for the city and port of Melbourne. The strategy for the defence of the port of Melbourne at this time was based on a number of shore batteries inside Port Phillip Bay.
In addition to coastal fortifications, this type of gun was also typically fitted to smaller British ironclads and as the secondary broadside armament on larger battleships. The primary ammunition was solid Palliser shot, primarily employed for Armour Piercing work. Initial design was 1865 and 3 successive upgrades were implemented