Right now, the cheapest time of year to take a flight to Apia is December/2020. You will pay the most in March/2020.
Virgin Australia and Polynesian Airlines have non-stop flights from Australia to Apia.
Fiji Airways is the cheapest airline to fly to Apia from Australia.
The best airline to fly to Apia from Australia is Air New Zealand. Virgin Australia is the best if you need non-stop flights.