In late May and early June 1942, a group of five Imperial Japanese Navy submarines made a series of attacks on Sydney and the nearby port of Newcastle. On the night of 31 May – 1 June, the submarines launched three "Ko-hyoteki"-class midget submarines against Allied shipping in Sydney Harbour. A torpedo intended for the cruiser exploded under the depot ship , killing 21. On 8 June, two of the submarines shelled Sydney and Newcastle, with little effect. In response, the RAN instituted convoys between Brisbane and Adelaide. All ships of over 1,200 tons and with speeds of less than were required to sail in convoy when travelling between cities on the east coast.