Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from San Francisco to Gold Coast as you normally would.
Due to multiple factors, expect to potentially pay more when booking in December. For low rates, May can be a great choice for travel to Gold Coast. Flights to Gold Coast from San Francisco in May are about $1,070 on average during May, but can be found for as low as $824.