It depends. In summer in the north of NZ where I live, water temperature can reach 22 degrees C (I've actually witnessed 26 degrees C way offshore) and we get an influx of tropical visitors including turtles, whale sharks and mantas. They are unusual sightings though. Fishermen get excited by the arrival of tuna and marlin. It is classed as sub-tropical. Visibility can be 25 metres, but the sea is rich in microscopic life, so visibility 10-15 metres is more normal. Obviously this depends where you dive. If you're in a harbour after heavy rain, visibility can be less than 2 metres. There's lots of fish including sharks, but it's rare to see sharks unless you are spearfishing, then you see too many! There's a few wrecks dotted around the north too. Water temperature in winter falls to below 15 degrees C in winter and there isn't as much life around. It can certainly be cold and dark! Whether winter or summer, the weather and sea conditions are highly unpredictable. You'll often get a good day in winter and a terrible day in summer. The Pacific Ocean is treacherous and it is essential to pay close attention to weather and sea forecasts. The further south you go in NZ, the colder it gets in general. Most diving is done on the east coast as the west coast is pounded almost relentlessly by heavy swells.
Australia is a huge country, a continent. You can experience everything from the conditions I've described in NZ (and colder!) in the south to the tropics in the north with everything in-between!
I'd love to visit you in NZ to dive there. Maybe one day