I’m not sure how Holden can claim that the car is now subject to the extended warranty (which is an aftermarket one from the third-party supplier) when your car should, according to Holden itself, be covered by the brand’s factory seven-year/unlimited km warranty. Since your car is still within that time limit, I would have thought the factory warranty applies and would cover the differential. The catch was that this seven-year warranty was only offered up to a cut-off date (March 31, 2018) but even after that date, the cover reverted to five-years/unlimited km, so your car should still be covered.
Holden only applied this seven-year warranty to 2018 ZB Commodore and Equinox models, but since the former is what you own, I can’t see how the factory warranty doesn’t apply. Keep trying and if you don’t succeed, the ACCC might be fairly interested in your case.
The difference will be, of course, if the differential problem has been caused by external factors such as a lack of servicing or you using the car to tow excessively heavy loads. But otherwise, the car should be covered against failures of this sort.
As for extended, aftermarket warranties, they’re really a huge rip-off in most cases. They are carefully worded to avoid the insurer paying out on anything actually likely to go wrong with the car. They also tie you into dealership servicing and, often, over-servicing, all in the name of lightening your wallet further. Forget them.