While reading some stuff in my ‚reed warbler book‘  last night, I suddenly remembered that there was a sighting or rather a ‚hearing‘ of a reed-warbler on an island where no such reed-warbler was known to exist, and I was quite sure that this was one of the Cook Islands but could not find any mention of it.
But then i found it and it was one of the Austral Islands, namely Raivavae, where a reed warbler was recorded in 1968, and it was apparently indeed not seen but heard only, but it was at least identified as being a reed-warbler.
The island of Raivavae has no surviving endemic land bird species today, but of course did have some of them in the past, among them very, very, veeery likely also a reed-warbler species, and in my humble opinion this appears to have survived until the mid 20th century at least.
So, I’ve checked my ‚usual suspects‘ and found some furter informations, but not really that much, unfortunately.
Te Manu: Bulletin de la Société d’Ornithologie de Polynésie. Nr 24. September 1998:
„Une espèce non identifiée d’Acrocephalus a été notée à Raivavae en 1968 mais n’a pas été retrouvé en 1990 (Seitre et Seitre 1991) et pouvait donc être un oiseau erratique.„
„An unidentified species of Acrocephalus was recorded at Raivavae in 1968 but was not found in 1990 (Seitre and Seitre 1991) and could therefore be an erratic bird.„
This record obviously is mentioned by D. T. Holyoak and J.-C. Thibault in 1984  but I was not able to read it myself, however, I’m rather convinced that the ‚erratic bird‘ more likely is meant to be what in German is called a „Irrgast“, a migratory bird that appeared on the island inadvertently while flying from one point to another.
But are there migratory reed warblers flying over the Austral Islands? No, because if they fly from north to south or back, they just do not cross the middle of the Pacific Ocean since the migratory reed-warbler species only inhabit the continents of the ‚Old World‘ and those inhabiting the Polynesian islands do not migrate, as far as I know.
In chapter 7 of David W. Steadman’s ‚Extinction and Biogeography of Tropical Pacific birds‘ from 2006  it is mentioned as Acrocephalus vaughani and as a ‚M, modern record‘ from Raivavae. How could I actually miss that until today?
I personally are rather sure that this single record from the island of Raivavae is indeed the last record of a former existing population of native, probably endemic, Raivavae Reed-Warblers which now join the ever-growing list of extinct taxa.
Here is a little update, in the meantime I was able to finde the original work by D. T. Holyoak and J.-C. Thibault from 1984.:
„A. v. sous-espèce ?
Raevavae: des fauvettes furent entendues par Lacan (J.-L. Mougin, comm. pers.) en mai 1968.“ 
„A. v. subspecies?
Raevavae: warblers were heard by Lacan (J.-L. Mougin, pers. comm.) in May 1968.„
 D. T. Holyoak; J.-C. Thibault: Contribution à l’étude des oiseaux de Polynésie orientale. Mémoires du Muséum national d’histoire naturelle 127(1): 1-209. 1984
 David W. Steadman: Extinction and Biogeography of Tropical Pacific Birds. University of Chicago Press 2006
 David Pearson; Peter Kennerly: Reed and Bush Warblers. Christopher Helm 2009