Here's a map of South-east Asia, showing levels of corruption (from the Independent). Australia in the bottom right is yellow, so it fares well. Malaysia is not too bad, being orange and ranked 50th out of 175. Indonesia fares less well (ranked 107), and Cambodia is even worse (at 156).
But what about Brunei? Unfortunately, there is no data on Brunei – it is the two white bits on the north of the island of Borneo, as shown in the black ellipse:
It is a bit of a mystery why no data is included for Brunei. Even North Korea is included (it is ranked 174th, second last, and only Somalia is worse). Perhaps there really is some data, but the map maker thought Brunei was too small to include the data on the map. Maybe that is the answer, as there also seems to be no data for Singapore.