What role did Australia play on the attack on Sydney harbor?

In the late afternoon of 31 May 1942 three Japanese submarines, I-22, I-24 and I-27, sitting about seven nautical miles (13 kilometres) out from Sydney Harbour, each launched a Type A midget submarine for an attack on shipping in Sydney Harbour. The night before, I-24 had launched a small floatplane that flew over the harbour, its crew spotting a prize target - an American heavy cruiser, the USS Chicago. The Japanese hoped to sink this warship and perhaps others anchored in the harbour. Read the rest of the story at : http://www.ww2australia.gov.au/underattack/sydharbour.html