During the Battle of Sydney Harbour (often referred to as the Attack on Sydney Harbour) on 31 May and 1 June 1942, "Steady Hour", having received orders at 3:10am, got underway from her mooring in Farm Cove and proceeded to patrol the harbour towards the boom net. At approx. 5:00am, her sister ship HMAS "Seamist" attacked midget submarine M-21 in Taylors Bay. Steady Hour proceeded to the scene and conducted further searches. At approx. 6:40am, a 'contact' was obtained by another sister ship, HMAS "Yarroma," on which "Steady Hour" proceeded to drop further depth charges. The submarine was later salvaged. Whilst credit for its destruction was initially given to "Steady Hour", it is now accepted the submarine had been destroyed by the earlier attack of HMAS "Seamist".