Why hadn't Chinese, Japanese or other East Asian nations conquered Australia prior to the British? Or had they?

If you read the book The Fatal Shore by Robert Hughes you'll find that the Chinese did visit Australia several times but did not find any incentive to settle there. Indeed most of their history shows the Chinese as less of an outward-expansionist nation and more of an inward-protectionist country (for lack of better terminology). Chinese expansion throughout history has almost all been focused on moving from the coast inland, instead of outward via the sea.

Although in a way of speaking, China did have a colonial empire, although it was very different to Western colonies. They had no need to cross seas when there were many different settlements within Central China (eg: the Uyghurs). The Chinese would take land from the non-Han (NH) people and then either assimilate the NH into their own culture or replace the NH with their own people. Another reason is the continual threat of China's own land neighbours (Mongolia, Vietnam and Thailand for example). Because of these two factors, the Chinese had little incentive to colonize outwards through sea.

To also understand why the Chinese didn't colonize outside their own borders, you should understand the basic reasons for European colonization. Trade and resulting wealth. Obtaining items not present at home and then selling a portion of it to make money. Europe was poor in this area, where they lacked luxurious goods such as silk (China) and spices (Middle East and India). The rise of the Ottoman Empire (Turkish) meant the disintegration of the Silk Road, so new strategies were developed.

Europe had nothing apart from precious metals to offer the Chinese. The Asians in general had all the luxurious goods they needed, and so they saw no reason to spend their wealth on fleets to colonize lands far away from them. While a handful of Chinese dynasties moved outwards to sea (notably the Song and Ming after the push of the Mongols, and the discovery of Taiwan and the Ryukyu islands of Wu during the Three Kingdoms), these intentions were not followed through later on and so mostly China kept to itself.

As for other East Asian nations, I can't say, but if I remember correctly Japan began their attempts to colonize in the 19th/20th century, though it eventually failed.