Pufferfishes, Porcupinefishes, Boxfishes and Triggerfishes are all in the order Tetraodontiformes. This web page focuses on the familes of Porcupinefish (Family Diodontidae) which are well represented in Sydney and common on many dives.
Porcupinefishes are similar to pufferfishes in that they can inflate themselves, but what makes them special is that they have large spines which point outwards if inflated. I am pleased that I have never seen this behaviour because it means that the fish is distressed, so do not try to provoke the fish to do this - it will not be fun for the fish!
The Three-Bar Porcupinefish is encountered on many dives in Sydney, especially at night. They are usually quite entertaining and like to look at you with their goofy beak like teeth and big eyes.