A bit of history on the US regs first, in simple terms. Part 135 ops= above 10k for crew, part 91 ops 12.5k.
Why the difference? It was lobbied by a major group back in the early 70's for the 12,500' for P91 ops to be able to pop up and over mountain ranges in the west of the country. Not stay there all day. This was to save the impost of expensive gear on the private operator. Part 135 is not so accomodating.
Recently, during an FAA FAAST team webinar, FAA personnel and the CEO of AEROX did a great presentation explaining the history and the science behind when you should use it. For most people, even fit and healthy folks, there is a significant cognitive benefit above 8000' and they were recommending despite what the regs say, if you are flying above 8000' for an hour or more you really should be using supplemental O2. For flight at night, vision is greatly enhanced as well as cognitive ability and the recommendation was 5000' and above at night.
I can personally attest to the statements made in that webinar are true as that is exactly consistent with my own testing 10-11 years ago. Myself and most of my flying friends all have O2 systems now. It really is a cheap legal performance enhancing drug!
This link is the same material presented by the "Aerox Dude", this time however for the Cessna Pilots Assoc. It is a long watch but worth doing, especially some of the slides showing FAA data.
Search YouTube for "Complete Guide to Supplemental Oxygen Use for General Aviation "
Also consider the recent Hypoxia event where the pilot should have been using O2 the whole flight. ATSB have a good report on the Caravan that sailed over Brisbane and out to sea.
There are two major suppliers, AEROX and Mountain High. Both are represented in Australia and the Aerox dealer carries stock here.
Dick, I believe that while CASA should do in this instance leave the current rules in place, basically Part 135 style rules. What they mean or should mean is "fitted" includes a portable system such as Aerox or MH.
I assume you have a portable or built in system for your plane, and that you probably would/should be using it above 8000' anyway as the health benefits are well worth it, especially as we get older. What do you have if any, and how do you use it?