Most protected is probably Neiafu Harbour in Vava'U, Kingdom of Tonga
. But cyclones can and do strike there; last major one was 31 December 2001. No marinas
there but many moorings available. In Anchorage #11 there are moorings placed by the owners of The Ark Gallery. This anchorage is not nearly as protected as Neiafu Harbour but the moorings are superbly grounded.
I would not suggest anywhere in Polynesia during cyclone season as safe. Cyclones are not predictable.
Amel SM2 #387
currently lying Auckland