Olla Nick!
I'm in Geelong, so hit me up for a dive when you get here and I can show you the ropes around here, some good spots, etc...
5mm is a good suit for year round here. Water temps in the bay are currently at about 12 degrees, and will drop to about 10, so its not much warmer than the UK...
Though based in Geelong, you'll likely be doing mostly south bay and ocean diving, so it stays about 11 - 12 degrees over winter, and spikes up to 22 - 24 degrees over summer, where a 3mm will be more than enough.
Your 90cm gun is probably going to be better suited than the 70. Most guys seem to use the +- 1m guns, and something a bit bigger if going out in the blue. I use a 110cm rail gun, and do find it a bit unmanageable at times, particularly in shallow.
Viz here can range from blackout to 30+ meters (though rare).
Dont worry about sharks. I've never seen a 'dangerous' shark whilst spearfishing in these waters. You're most likely to encounter big smooth rays while spearfishing from shore, as far as unsettlingly large sea creatures go, but they're going to be more interested in the fish strung on your float line than you. A poke with your speargun is generally enough to send them on their way if they're getting a bit friendly.