The only way to reach Niutao is on one of Tuvalu's two government inter-island vessels, the Manu Folau or Nivaga III, on the long northern run from Funafuti. The voyage takes around a day each way, with stops at the other northern islands depending on cargo. Schedules are irregular — sometimes only once a month — and depend on weather and demand.
There is no airstrip on Niutao and no other route in or out. Arrange your visit through Tuvaluan contacts in Funafuti well in advance and be prepared for delays in both directions: the ship may not return on the date it was supposed to, and you may end up on the island for considerably longer than planned.