Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Sydney to Belize City as you normally would.
Sydney Kingsford Smith (SYD) is the airport you will fly out of if you book a flight from Sydney to Belize City. You should not have trouble finding flights to Belize City from Kingsford Smith; the airport handles an average of 0 outbound flights to Belize City per day. You should also have plenty of airline options to choose from since 1 airline has flights from the Sydney airport to Belize City. It is only a 7.5 km journey to Kingsford Smith (SYD) from Sydney’s city centre.