The classic Japanese war plan against the United States in the Pacific involved using Japan's island possessions throughout the Central Pacific as bases for Japanese air and light surface forces to attrit and wear down the U.S. Pacific Fleet as it sailed west, likely for the relief of the presumably besieged Philippines. Once the Pacific Fleet had been worn down, the Japanese Combined Fleet could engage it at something closer to parity, where--the Japanese hoped--that the superior quality of individual Japanese ships would overcome the numerical superiority of the American fleet and a decisive naval victory for Japan. Faced with the need to rebuild their fleet and attack--again--into presumably strengthened Japanese defences, the Americans would opt for a negotiated peace in Japan's favor.

However, when the actual Pacific War came, the Japanese were faced with a conundrum. Rather than attacking the Philippines then awaiting the American counterattack, the IJN was to take a more aggressive posture, campaigning south to take the British and Dutch holdings in South East Asia. This in turn presented the Japanese with a conundrum: if the American counterattack came too quickly, they could catch the Japanese fleet out of position, scattered throughout South East Asia, and leaving the Japanese at a critical disadvantage. The attack on Pearl Harbor thus had a number of goals: 1) to cripple the U.S. Pacific Fleet at the outset, buying the Japanese time to complete their conquests in South East Asia before the American counter attack could come; 2) potentially knock the United States out of the war early with a highly visible destruction of the key symbols of naval power.

As such, so long as the Japanese war plan involved seizures of South East Asia, there was a pressing need to ensure the American fleet was not able to intervene immediately. The Pearl Harbor attack provided an opportunity to both prevent the American fleet from attacking immediately, while also--potentially--knocking the U.S. out of the war before it could even be truly involved.

In that respect, while further conquests in the South Pacific were useful in that they created further layers of defence between Allied forces and the vital resources of Java, Sumatra, Borneo, and Malaya, and increased pressure on Australia, they would not likely be of great use in creating further barriers against an American thrust to the Central Pacific, which would fall against islands that the Japanese had occupied since 1918. In addition, it is highly likely that the Japanese simply would not have the logistical capacity to fully occupy Australia. Potentially, they could raid or even occupy ports or bases in northern Australia, but the Japanese logistical infrastructure was already intensely strained by the needs of the war in China, the additional burden of garrisons and bases throughout the Pacific, and the additional need to move resources extracted from South East Asia north to the Japanese Home Islands. Being drawn into another continent scale conflict in Australia would likely have been too much for Japan, and so an occupation of Australia would have been off the cards. It would have been much simpler to interdict the supply lines from Australia to the United States thus reducing the ability of the Allies to use Australia as a forward base.

So, to summarise: it's questionable whether the Japanese could seize Australia, and more importantly further seizures in the South Pacific would not have been effective in the goals that the attack on Pearl Harbor was meant to accomplish.

I've gone into some more detail about why the Japanese were so certain that the United States would intervene here.