This is a crucial aspect of your equipment. This is what determines how long you can last diving and how efficient your dives become. It is strongly recommended to use GKA or open cell neoprene wetsuits especially around colder water temperatures.
Note: A 3.5mm wetsuit is a great all rounder for Australian waters. It is always recommended to be warmer in a suit then feel cold. If you feel warm you can let some water in. However, if you start to feel cold then your diving efficiency drops dramatically, and you will no longer be able to dive.
Remember, heat escapes from your head, hands and feet. These three parts must be the warmest part of the outfit and keep this in consideration when purchasing the right thickness.