Why were Australian soldiers taken prisoner by the Japanese during WWII?

Australian soldiers were taken prisoner by the Japanese in WWII because the Japanese military was able to overwhelm the places that the Australians were defending.  They did not want to simply massacre all the Australian soldiers so they took them prisoner instead.

At the beginning of WWII in the Pacific, the Japanese military was much better prepared for war than any of the Allies were.  The Japanese were able to overwhelm the Americans in the Philippines, the British and Australians in Singapore and Hong Kong, and other Allied forces in the region without much trouble.  The British and Australians (and Indians) were more numerous but were poorly trained as the best-trained troops were off fighting in the European theater.  For these reasons (and some others, such as the mistakes made by commanders in the field), the Japanese were able to defeat the Allies very quickly in most parts of the Pacific.  Therefore, they were able to take many Australian prisoners.

When the Japanese won so easily, they had choices as to what to do with the people who surrendered.  They could either slaughter them or take them prisoner.  They took them prisoner, presumably for two reasons.  One reason was that they did not want to kill that many people in cold blood in ways that could not be hidden from the world.  The other reason was that they wanted to take prisoners and use them as forced labor.

Thus, the Japanese took Australians as POWs in WWII because they were able to defeat the Allies very easily in the early stages of the Pacific war and because they did not want to simply massacre those soldiers who surrendered.