In general, carriers won't allow you to board your first flight in the itinerary unless they're certain you've got the visa you need for your final destination. Which makes intuitive sense - they don't want pax getting stranded mid-stream if the visa didn't process while you're in the air. Let us know how you get on though. In the best case your AU visa will process quickly, but if you do succeed in sweet-talking CX into carrying you to HKG without it that would be an interesting data point.