Right now, the cheapest months to take a flight to Guangzhou are March/2020 and February/2020.
Qantas Airways, China Southern and XiamenAir fly direct between Australia and Guangzhou.
China Southern is the cheapest airline that flies to Guangzhou from Australia.
Singapore Airlines is the best airline to fly to Guangzhou from Australia. For non-stop flights then Qantas Airways is best. The best low-cost airline is AirAsia.