I don't see GM Aus or anyone affiliated being helpful as like you've mentioned, they would have no vested interest in looking after somebody who is essentially not their customer. Unfortunately it is the inherent risk with choosing to buy overseas, I had the same experience a few years ago, and fortunately the UK retailer was really helpful.
I feel like it would be a pretty common issue with people choosing to buy from the UK considering how inflated the AUS prices are in respect to the UK prices.