Most government offices on Niuafo'ou are at Esia, but the Telecom office and Civil Aviation offices are at Sapa'ata.
There are no official accommodations on Niuafo'ou, though some of the locals will accept paying guests. The Catholic Mission also takes guests.
The Real Tonga agent on Niuafo'ou should be able to arrange accommodation, and the airline office in Vava'u will radio ahead to let them know you're coming. Camping on the crater is possible.