I have been looking at the information from asa since yesterday and ersa is instructive.
A flight into NFI must lodge it's plan with asa.
NFI, is FIA in all paperwork [DAP's and ersa], with a release on descent NFI from the overlying oceanic airspace [FIR - NZL and FIJI] and the ersa says:
I believe the AIP - ENR also is instructive at ENR 1.10.1.1 - This makes it an ASA responsibility even as it is octa below the FIR and the PROC would be "On leaving 13000 [1013 HPA], descent octa NFI" the control then passes to the pilot and advice by HF and VHF to BN centre and NFI unicom.
An asa responsibility, not NZ and part of the submitted flight plan which would be Samoa - NFI - YMMB