Currently, December/2020 is the cheapest time of year to take a flight to Colombo. You will pay the most in November/2020.
SriLankan fly direct between Australia and Colombo.
The cheapest airline to fly to Colombo from Australia is Singapore Airlines.
Qatar Airways is the best airline to fly to Colombo from Australia. SriLankan is the best for direct flights. The best low-cost airline is Thai Lion Air.