Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Washington, D.C. to Rotorua as you normally would.
For a flight from Washington, D.C. to Rotorua, you will be flying into Rotorua. Rotorua (ROT) is just 4.4 mi from the center of the city. Rotorua is the only Rotorua airport and it handles 0 inbound flights from Washington, D.C. per day. 0 airlines have flights to Rotorua from Washington, D.C. on a regular basis.
When deciding which route to take from Washington, D.C. to Rotorua consider Baltimore/Washington - Rotorua, Washington, D.C. Reagan-National - Rotorua or Washington, D.C. Dulles Intl - Rotorua as they are the most popular.