Hi, I live in South Australia and have the following comments.
While South Australia (SA) has large populations of both Australian sea lions and long nosed fur seals (i.e. the local name for New Zealand fur seals), the only places where tours are offered by licensed operators and where one can get it in the water to see sea lions are Baird Bay on the Eyre Peninsula and Hopkins Island in Spencer Gulf. There are no tours that focus on fur seals. Marine mammals are protected under state law and there are statutory limits on how close one can get to a marine mammal (i.e. 30 metres). Fur seals are common within areas of the state that are populated by humans and do occasionally swim up to divers to check them out.
A number of stores and other service providers offer leafy seadragon tours to places such as Rapid Bay and Victor Harbor. I suggest that you look at the Facebook pages for the following selection (i.e. some tours don't operate in July) and use the Facebook messaging service to make contact in the first instance - Adelaide Scuba, Diving Adelaide, Divers Service, Downunderpix, NB Scuba, The Dive Shack and Underwater Sports Diving Centre. Some tours will include transport to and from the site while others include a pick-up service.
Yes, you can dive with nitrox if you want to. The following stores in Adelaide advise on their websites that they sell it - Adelaide Scuba, Underwater Sports and Divers Service. In my opinion, most dive sites are too shallow to justify the expense.
By the way, July is winter time and the water will be cold, i.e. less than 15 degrees C.
