Water temps in Port Phillip Bay certainly
seem like 10C in winter... but are actually around 14C during September, the coldest month.
Port Phillip Sea Temperature | September Average | Australia | Sea Temperatures
Chilly enough even in a double layer 7mm suit...
Now regarding regulators... some very good points already raised but my 2BAR:
For piston regulators I am a fan of Atomic Aquatics, well designed and breathe very easily... the AFC feature really does work and their patented second stage seat saver means service intervals can be longer than other brands. I'd look at a Z3, they breathe the same as a T3 for a much more economical price, or a B2 if you want the first stage swivel and more Ti internals in the second stage.
Dive Regulators: The Z2/Z3 Regulator from Atomic Aquatics
HOWEVER... getting Atomic gear serviced in Melbourne can be exorbitant!
I was quoted $300 for a full service on an Atomic Aquatics B2 enviro sealed first and second stage with Z2 octo
If it was a T2-T3 they wanted another $150... apparently just because everyone expects a Ferrari service to cost more than a Ford!
I note this dive shop has since closed its doors.
This was a major instigator in learning how to service my own gear (bought all the correct tools including the lube repacking tool and I'm STILL in front)... my family has three sets of Atomic gear!
Special Tools for Atomic Aquatics® Regulators
Atomic service costs may have changed with the recent transfer of Australian distributorship from Aquanaut to Australian Underwater Products (AUP) but they insist that:
It is AUP policy that only people associated with a dive store; who have recently attended (within the last 3 years) an accredited service technicians course are eligible to attend the Atomic & Zeagle service course provided by us.
So I don't see that changing... and it will be a major consideration if you want to buy Atomic.
Recently I have bought and serviced two s/h Aqualing Mikron reg sets (orginally for travel regs) and have been impressed with both the advantages of a relatively sealed diaphragm first stage and the compactness and ease of breathing the second stage offers. Exhaust bubbles from such a tiny second stage can be distracting but the impressively small pack-down size outweighs that if you do much dive travel.
So you may like to consider the Aqualung Mikron or the Titan/Legend for a larger 'form factor' with less bubbles past your ears.
Either way, there's some great diving in and around Port Phillip Bay (gotta check out the WW1 subs and Leafy Sea Dragons!) so have fun whichever way you jump on gear choice!