Two days after the attacks on Sydney and Newcastle, a Japanese submarine fired on the Age, an Australian coastal steamer travelling from Melbourne to Newcastle. An hour and a half later the merchant ship Iron Chieftain signalled that she had been torpedoed about 43 kilometres north-east of Sydney, in the same area. The Age reached Newcastle safely but the Iron Chieftain, loaded with coke for ship building in Whyalla, sank in about five minutes. Twelve of the crew, including the captain, went down with the ship while another 25 crewmen abandoned ship and landed in lifeboats on the beach at The Entrance.
After these attacks, the ports of Sydney and Newcastle were temporarily closed to outward traffic. The Naval Board suspended almost all merchant ship sailings from ports between Brisbane and Adelaide. Convoys were introduced in an attempt to protect shipping from attacks. There were surface and air searches for enemy submarines, and ships at sea were warned to zig-zag widely.
On 4 June, just south of Gabo Island, off southern New South Wales, a Japanese submarine attacked the steamer Barwon with torpedo and gunfire, the damaged steamer later limping into port. In the late afternoon on the same day, the Iron Crown was torpedoed in the same area and sank immediately. There were five survivors from a crew of 42. The Japanese submarine I-24, the 'mother ship' of one of the midget submarines that had raided Sydney Harbour, was suspected to be responsible. The I-24 reputedly sank three ships south of Sydney during this period.