And by the way I would guess the fuel up there is possible from BP Kwinana Refinery which has a lower grade alkylate than is required for 100LL so it will be AVGAS 100 (green stuff) and with a higher TEL content.
Jabawocky, it's highly unlikely the Avgas in Arnhem Land comes from Kwinana. There is no refinery in the NT and the vast majority of fuels used in the NT are imported.
It would not be even remotely economic to refine avgas at Kwinana and ship it or truck it to the NT, because the Asian refineries can refine and deliver fuel to the NT for substantially less than any Australian refinery can even produce it.
The avgas in the NT is refined overseas (usually Singapore) and imported in bulk and stored at the Darwin Vopak terminal, from where it is distributed throughout the NT.
The following report is a few years old now, but about the only things that have changed since 2009, is a couple of Australian refineries have shut down due to age and economic inefficiency, and imports of refined fuels (refined specifically to meet Australian Fuel Standards), have increased.
Australias petroleum import infrastructure