HMAS Platypus was a submarine depot ship and base ship operated by the Royal Australian Navy (RAN) between 1919 and 1946. She was built by John Brown and Company at Clydebank in Scotland, and launched on 28 October 1916
Ordered prior to World War I to support the Australian submarines AE1 and AE2, Platypus was not completed until after both submarines had been lost, and she was commissioned into the Royal Navy from 1917 to 1919, when she was commissioned into the Royal Australian Navy at Portsmouth, under the command of Commander Edward C. Boyle VC RN, as a Submarine Depot Ship for six J Class submarines transferred as a gift from the Admiralty to the Royal Australian Navy.
In February 1920 Platypus proceeded to Port Phillip where a Submarine Depot had been established at Geelong.
In May 1922 the Naval Board decided to abandon the policy of maintaining a Royal Australian Navy Submarine Service. The three J Class boats remaining in commission were paid off, and on 1 July 1922 Platypus proceeded to Sydney.
Meanwhile, in 1924, a Five Year Naval Development Programme had been approved by the Australian Government, which included the re-establishment of a Royal Australian Navy Submarine Service with a flotilla of six boats.
Two Royal Navy Odin Class submarines were initially ordered. Named Otway(I) and Oxley(I), the submarines reached Sydney on 14 February 1929. Platypus returned to Sydney from a cruise in Queensland waters the following day. She paid off on 31 March 1929 to recommission in her former role as a Submarine Tender.
On 12 July at Sydney she paid off as a Submarine Depot Ship and on the following day recommissioned as a Destroyer Depot and Fleet Repair Ship. Operating with the Fleet, mainly in home waters, she served in this role until 1929. The reconstituted Royal Australian Navy Submarine Service suffered from the outset from the world wide naval retrenchments beginning in 1929.
Platypus paid off on 15 August 1929 and the following day commissioned as HMAS Penguin and operated as a depot ship until 26 February 1941 when she recommissioned as HMAS Platypus to resume seagoing service as a training ship.
In October 1932, a mutiny occurred aboard Penguin. The mutiny was in protest of the decreases in sailor pay and conditions: Depression-era cutbacks had impacted them harder than officers, as they had no avenues of protest. The ship's commanding officer was sympathetic, promising that he would forward their concerns to the Australian Commonwealth Naval Board and that they would not be punished. The Naval Board refused to consider their concerns,
Platypus continued in service as the Depot Ship at Garden Island, Sydney, under the name of Penguin until May 1941, when she proceeded to Darwin.
She was present in Darwin Harbour on 19 February 1942 when Japanese carrier borne aircraft made the first air attack on Australian soil. According to the official history of the Royal Australian Navy in World War II, ‘Platypus fought back hard, and though near-missed three times and with the lugger Mavie alongside her sunk escaped with damage in the engine room which immobilised her for some time.’
The ship assumed her old name and was relocated to Darwin, then Cairns for use as a base ship. After a refit in 1944, Platypus operated as a repair vessel in New Guinea waters until she was placed into reserve in 1946.
She remained in service as Base Ship, Darwin, until 1 January 1943 when she sailed for Cairns where she again served as Base Ship until May 1944.
At Williamstown, Victoria, on 12 June 1944, Platypus commenced a major refit and conversion of two of her four boilers to oil burning. The refit was completed in December 1944. On 5 January 1945 she left Sydney to proceed to New Guinea for service as a Repair and Maintenance Vessel.
Platypus operated in the Madang, Hollandia and Morotai areas until the end of November 1945, returning to Australian waters in December 1945. On 12 February 1946 she departed Melbourne for Sydney on her final sea voyage under her own power.
The wartime service of Platypus was later recognised with the battle honour "Darwin 1942-43"
Platypus paid off into Reserve at Sydney on 13 May 1946.
She was sold for scrap on 20 February 1958 to Mitsubishi Shoji Kaisha Ltd, of Tokyo, Japan, . In June 1958 the Japanese salvage vessel Tukoshima Maru departed Sydney for Japan with Platypus and the former Bathurst Class minesweeper, HMAS Dubbo J251/M251, in tow.