 In 1974 , Australia purchased 12 Westland Sea King Mk 50s as the Royal Australian Navy 's new ASW helicopter ; the Sea Kings replaced the aircraft carrier HMAS Melbourne 's complement of Westland Wessex HAS31 in the following year . Early operations were troubled by a series of accidents . Between October 1975 to May 1979 , four aircraft were lost in accidents , the primary causes were the loss of oil from the main gearbox . 
