Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Hong Kong to Tonga as you normally would.
Related info for your journey
Useful info, stats and facts about Hong Kong to Tonga flights.
Hong Kong Intl Airport is HKG and Nuku‘alofa Fua'Amotu Intl Airport is TBU. These codes are created by the International Air Transport Association (IATA) and airlines use them to identify the departing and arrival airports in the flight routes they service. Vava'u Lupepau'U Airport (VAV) are other airports near this flight route and their unique identifiers/IATA codes.