Hi,
I am a kiwi and I have done quite a few dives in australia. Mainly around sydney.
It depends where you are going in aussie. The water temp in sydney is about 16-17C in winter so a drysuit is nice. Melbourne and the south coast of australia is cold water diving so a drysuit becomes a lot more usefull. The water is not as cold as the uk tho.
If you are going to cairns then a drysuit is not necessary.
Australia is a big place. A semi is probably ok for diving in aussie but being dry and warm is a lovely feeling. If you are planning on going anywhere south I would get a drysuit.
Mike