What did the attack on Pearl Harbor mean for Australia?

The attack on Pearl Harbor meant America's navy would be unable to come over and defend Australia and other Asian countries from Japan's invasion. That was Japan's purpose when they attacked Pear Harbor, to allow a more easy invasion of Australasia without American interference.

Another Viewpoint:

The Japanese attack on Pearl Harbor meant that Australia's region had come under direct threat and Australia had to withdraw its forces from the war against Germany and bring them home. While Australia was attacked by air and submarine, the Japanese, having achieved their aim of taking over the resources of South East Asia, and being committed in China and the Pacific, had no capacity leftover to attack Australia, as much as we feared this might happen. The country went on a full war-economy and the reserve forces were called up for full-time duty. It became a base for the American army's campaign to retake the Philippines and contributed to preparations for the invasion of Japan.

An indication of Japan's inability to attack Australia was that the best force they could assemble to take over New Guinea was an infantry regiment and a scratch regiment of engineers and other troops - not one full division, while Australia had five infantry and three armored divisions even before the three AIF ones in the Middle East were brought home. The Japanese Navy was all for pressing on to Australia, but the Army gave a flat 'No' ass they had nothing spare - their troops were fully committed in Burma, South East Asia, China and the USSR border.