Koala's have only made rare appearances on "official" circulating coins. The was a one-off 50 cent piece a few years back that featured a koala along with a wombat and the was the NCLT $200 bullion coin from the 1980's. I can't think of banknote featuring a koala though. The koala might feature more frequently on the collector's crap that the Perth Mint churn out!
The kangaroo is far more common though. As the kangaroo (along with the emu) feature on Australia's coat of arms, any banknote that features the the coat of arms features a roo! Currently in terms of coins the kangaroo curently features in the default designs for the 50c & $1 coins. In pre-decimal days the roo famously featured on the half penny, penny, and as part of the coat of arms on the sixpence & florin