Diving around Marseille is usually very cool in August but it could also be windy.
In that case, try Carry on the west side or Hyeres near toulon on the east side:
Carry Le Rouet is protected from the north wind by the hills of the " blue coast" ( côte bleue"), but if the north wind blows, my advice is avoid diving because the watter temp can drop down from 25°C to 14°C, the viz also gets crappy.
Anyway, Carry is a good place and this dive op is very wellcoming, he is not situated on the harbour but he carries the tanks and gears.
The viz is usually better in Marseille than around Carry, here he is :
Plongée Passion Carry-le-Rouet Marseille
On the East side, THE spot is around Hyeres and the islands of Hyeres ( îles d'Hyères). Fantastic wrecks ( the best are quite deep : about 50 m). If you can't or don't want to go that deep, there are very nice dives near the islands, Port Cros island is a protected area ( nature reserve) where the wild live is abundant.
This place is probably the best on the french southern coast, here's a PADI dive center :
Destination Plongée // Hyères // var.
Marseille eventually, is also a must for diving, despite this northern wind they call " mistral".
If you are lucky, it won't be windy and, the watter temp will be cool ( 25°) and the viz more than 20 m. They also have deep wrecks and very good diving spots .
Two adresses I heard about but never tried myself. Both professional dive op and PADI certified :
Accueil au Centre
Evasion Sport, 20 ans d'expérience dans la vente de matériel et accessoires de plongée multi marques.
You have to know that in France some dive center are only FFESSM certified ( a bit like a local BSAC), they could not accept an AOW as such and consider you like an OW. So you'd better choose the PADI dive ops ( I have to say that I'm not PADI myself).
You also have to know that here you must have a medical certicate saying that you're fit to dive. The dive op is reponsable of you and he must make sure that you have a medical agreement...this is the law.
I come back myself from Australia and I hope you will enjoy your trip as we did.
Where do you live in Oz ?
(apology for my surprising english...Shakespeare, will you ever forgive me ?)