New Guinea is one of the centres of diversity of Benstonea (Pandanaceae), a genus distributed from India to Fiji. Ten species were previously recognised on this island and further field observations, accompanied by the study of available herbarium material have brought new insights into species delimitations within a group of caespitose species with a solitary terminal infructescence. The taxonomical identity of Benstonea odoardoi is elucidated and is considered here as a synonym of Benstonea lauterbachii. Three new combinations and a new name—based on names of Pandanus species previously treated as synonyms of Benstonea odoardoi—are proposed for four distinct species belonging to this group of caespitose species and restricted to Indonesian New Guinea and Papua New Guinea. Finally, Pandanus bintuniensis is here considered as a synonym of Benstonea permicron.

Halmahera is the largest (c. 18,000 km2) island of the Moluccan archipelago, but naturalists have only sporadically visited Halmahera and it has remained very poorly explored botanically. However, an intensive botanical inventory project was undertaken between 2012 and 2014 in part of the island to inform flora biodiversity management for certain proposed mining activities. This effort has contributed over 3600 plant collections and nearly doubled the number of Pandanus Parkinson (Pandanaceae) specimens (bringing the total to 55) available for Halmahera. After careful examination of all available material and comparison with other material from the region, we are able to present the first overview of the genus for the island. We have identified ten species from the island of which three are new to science and not known elsewhere, while the other seven are all representatives of species already described from other localities. The new species are formally described here as Pandanus beguinii Callm. & A. P. Keim, Pandanus benstoneoides Callm., Buerki & Phillipson and Pandanus halmaherensis Callm. & A. P. Keim. The new species are provided with notes on their respective morphology and known distributional and ecological ranges, line drawings. Those three new species are assigned a preliminary status of Endangered following IUCN Red List Categories and Criteria. All ten species are illustrated with colour photographs and a key to the species is provided.

Benstonea (Pandanaceae) was circumscribed to include 57 species formerly placed in the genus Pandanus. Field observations, accompanied by the study of available herbarium material have brought new insights for the delimitation of certain problematic species, especially in the difficult group of species characterized by an axillary infructescence on a short peduncle covered by prophylls and the abscission of the basal portion of the drupe at maturity. New combinations, based on names in Pandanus previously treated as synonyms of Benstonea stenocarpa, are proposed for three distinct species of this group from Halmahera (Indonesia) and Papua New Guinea. The identity of Benstonea celebica, endemic to Sulawesi (Indonesia), is also elucidated and an epitype is designated for this species.