NOTE - THIS PROTO-PAGE IS STILL IN THE EARLY STAGES OF CONSTRUCTION!
This name possibly originally functioned as a generic label for Cyathea and related genera. The Sphaeropteris and Alsophila species were formerly classified under the genus Cyathea (and still are by some taxonomists); the New Zealand species, which include the iconic "silver fern", Cyathea dealbata, remain under that heading (for the moment at least).
Another possible Tongan cognate is recorded in R&W (p. 421) with the note "Endemic to Tonga" as Gaillardia aristata. This is probably a misprint, as G. aristata, a member of the daisy family [Asteraceae], although widespread in the tropics, is native to the Americas and, obviously, does not fit into the otherwise general use of this name for tree ferns or very large ferns.
The use of this name in Rarotonga to denote Cyathea dealbata, as in Aotearoa, is recorded in Pollex, but this will be a recent acquisition to both the flora and the vocabulary, as C. dealbata is endemic to New Zealand. The Samoan referents are marked in R&W (p. 381) as requiring further investigation to establish their validity; the sound correspondence to the Tongan and Maori reflexes of *Ponga is also irregular. However, the meaning is intriguing, especially if the Samoan term is in fact the "ancient name for oliolï, Cyathea spp." as one of their sources is quoted as having stated. The Cook Islands Biodiversity Database has an entry for Cyathea decurrens "Budding Tree Fern", native to the Cook Islands, with the Rarotongan name panga - thus possibly cognate with either or both the possible Samoan, or established Tongan and Maori, reflexes of *ponga. One of the species names listed by R&W, Cyathea vitiensis, is a synonym for Sphaeropteris lunulata (see Large & Braggins, Tree Ferns, p. 156).
Photographs of several of the tropical species bearing the name Ponga have been elsive -- Sphaeropteris lunulata is illustrated on the right; there is a photograph of Angiopteris evecta on the page for *Palatao (from which Proto Polynesian word one of the alternative names of this giant fern is derived). The photograph below, left, is of some Cyathea-like ferns, perhaps including S. lunulata, in the Fijian forest, and that on the right is of the New Zealand species, C. dealbata.