Due to multiple factors, expect to potentially pay more when booking in January. For low rates, May can be a great choice for travel to Naha. Flights to Naha from Darwin International in May are about $822 on average during May, but can be found for as low as $459.
When travelling, it's generally smart to pack in preparation for rain or any other conditions that may arise. The rainfall in Darwin is roughly greater than that of Okinawa. You can assume this to be the case over the next 23 days.
With an average temperature of 34 °C, peak weather can be found in October. As a traveller, expect to see changes in flight prices during that period. The temperatures in Okinawa are colder than in Darwin.