Australian banknotes are very difficult to copy and most people will never see a counterfeit note, but it can and does happen. Detector pens are available to run across the note, which will change colour if it’s fake. Or use the simple crumple test: Australian banknotes can easily be straightened once squashed.
However, there are more scientific ways to spot a dud banknote. Take a couple of minutes to read through the Reserve Bank of Australia’s Counterfeit Detection Guide, which explains what to look for on a banknote and what to do if you suspect you’ve received a counterfeit.