AC has a monopoly YVR/BNE & YVR/MEL nonstops & a virtual monopoly YVR/SYD(QF only seasonal few days a week).

Whenever there's a monopoly you'll pay a premium.

Look at options via LAX, SFO.

Fiji Airways is a fantastic airline & flights are nonstop daily LAX/NAN with good connections about 5 days a week to BNE. They also fly up to 3 days a week SFO/NAN at around the same time. Their long haul fleet is all newish A330's.

Have flown business class a number of times. It's much cheaper than the nonstops, plus if you pay for tickets with dollars, not points/miles they have what's called BULA BIDS, where you can bid on long haul flights, starting about AUD$865.

+ bonus in flying via Fiji, is you can stopover in fabulous Fiji which must be the happiest place on earth.

Stops of less than 72 hours mean you don't incur Fiji departure tax of FJD$200/adult (about AUD$135). Plenty of good 5 star accom within 20 mins cheap taxi ride from airport at Denarau(called an island but it's not really) + many more bit further + many resorts on islands.

On www.fijiairways.com you can book YYZ/NAN but better option would be to find a decent living travel agent in YYZ, who can book you're whole itinerary YYZ/BNE via either SFO or LAX & NAN.

IMO - Fiji Airways is a much better airline that AC, any U.S. airline, Qantas, Air NZ & probably on par with Virgin Australia.

The flights from NAN to either BNE or SYD are only just over 3 hours.

DATES OF TRAVEL
You mention northern winter which is summer in OZ. School holidays in OZ run from mid Nov-early Feb (every state in slightly different). Qld school holidays run from late Nov to late Jan for private schools & bit less for govt schools, so you need to be flexible date wise, as day or 2 difference can make a huge difference airfare wise.