It's pretty simple.
Aussies have "thicker is better" mentality. No logic really. They'll come around with enough time.
WRT to the "no logic", Australians would NEVER accept 1qt/1,000 miles as "normal" oil consumption, and the neutering of additive packs to prevent "normal" oil consumption from wrecking catalytic converters.
Holden jad an issue a few years ago when commondores were draining their sumps, as nobody could fathom going from full to destroyed inside a single OCI...when the country was accustomed to 1 litre/5,000 km being consumption worthy of concern.
Yep, 20W-50 reigned for nearly ever.
In the early/mid 80s, Shell introduced XMO 15W-30, and it quickly gained a reputation as massively increasing oil consumption (deserved) and wrecking engines (undeserved).
The hyper thick movement started around 20 years ago with the likes of STP producing 25W-60s and 70s, and selling them as "ideal" for outback Australia.