There is definitely kelp off the Victorian and South Australian coasts, but for serious kelp forests, you want to be diving off Tassie (especially the East coast I believe). That water is cold. If you just want to tick it off the list you could get away with a thick wetsuit with hood and gloves. You could do a short dive before the cold was intolerable. But if you actually want to enjoy it, and spend long enough to see the marine life in any detail, you really want to be in a drysuit.
Kelp forests aren't particularly deep (the 18m depth limit of an open water course is sufficient), but the water may not be particularly calm, and visibility not great. Your aim should be to be comfortable in cold water with a bit of surge and current, and not great visibility. Starting from a zero diving basis, I would suggest:
- Do your open water course locally, which preferably is somewhere with cool/cold water (Victoria, SA, Tassie, Southern NSW) and variable conditions. Open Water is rarely taught in a drysuit (except in Scandinavia) but the jump to drysuit diving is much easier if you're already comfortable in cool water and a thick wetsuit.
- Dive, dive, dive. Don't be put off by questionable conditions (but stay safe and within your ability and training limits). Along the way you'll probably do Advanced Open Water (the 30m depth rating isn't necessary, but the experience is good). Focus on buoyancy and getting comfortable underwater. Around Melbourne, the pier dives along the Mornington Peninsula are a great way to get experience. Shallowish reef and wall dives are great too.
- Learn to dive in a drysuit. This may involve a course, or you could learn from an experienced mentor/buddy.
- Find a good dive operator in Tassie that can get you to the kelp forests.
I'm in Melbourne and can recommend some good shops and operators here if that helps.