@rpmpam
I do live down under, so happy to assist where I can.
The most obvious thing is that Australia is about the size of mainland USA but only has a population of about 28 million - a bit smaller than Texas. Most of the population is crammed into coastal cities, only five of which exceed a million people. Both Melbourne and Sydney exceed 5 million, which I believe puts them second only to New York in size. Brisbane and Perth are well over 2 million each.
So there is extreme concentration separated by large distances. You can drive between Melbourne and Sydney in a day, but expect to take several days to get to Perth from the East Coast, or several hours by plane.
There are dozens of HiFi shops in Sydney and Melbourne, but I have not lived in Melbourne for several decades.
Used Gear: How is used gear traded in AUS? Is there an online platform specific to AUS or are Aussies using the same platforms as the US e.g. Agon, US/Canuck Audio Mart? If latter, how was your experience importing gear from North America including any info on tax, duties, tariffs?
In Sydney, two high-end stores stand out - Len Wallis Audio and Audio Connection. Both sell used equipment, possibly on consignment, and have websites or newsletters alerting subscribers. Stereonet is the (sort of) TAS or Stereophile, locally.
Australia believes in free trade, so there are no tariffs. There is a blanket Goods and Services Tax (GST) of 10% on almost everything. Mostly this is waived on low value imports (A$1,000) so you may be better off importing than buying locally.
Go To Retailers: Who are the knowledgeable and trusted retailers of high end HiFi gear? Is it typical to ask for a discount on expensive gear or is it culturally not a thing in AUS
In my opinion, everybody expects a discount, so ask for one. Culture shock: we do not tip. Everybody is equal and paid a decent wage.
Aussie Brands: Any made in Australia brands you would highly recommend?
Yes, if you are into $50,000 turntables.
Cables: Do companies like Nordost or Kubala Sosna make Australian power cables or are people using US/UK cables with adapters or power distributors?
I would never recommend downgrading (transforming) good 240-V Aussie power, or using adapters. In Australia your gear will need under half the current, and the plugs have flat faces for good contact. I would not import anything that does not run on 240-V AC, 50 Hz.
Aussie HiFi Clubs and Associations: Any local HiFi club, association, newsletter, forum you would recommend?
There is a Sydney HiFi club but I have not been to any meetings. I would recommend the Melbourne HiFi Show, but skip the Sydney one.
Other: Any other advice or pointers are highly appreciated
Happy to help where I can, but don’t expect great internet even in the capital cities. I mainly treat Sydney / Melbourne as one HiFi district when buying, but the world is your oyster. Just about everywhere else is equally far away, except NZ