According to the game files, for the decision "NZL_demand_islands" to be available, you need :
-To be free OR to have Dominion autonomy level
-To have completed Maori Volunteers focus
-To be in a faction with a country that possesses the involved territory and is NOT one of the following :
HAW, TAH, GUM, FSM, SAM, SOL, FIJ, NZL (I think these are all the island countries of the pacific, so you cannot ask them to hand over themselves if they were released by the colonial powers prior to your demands ; and you can't ask yourself to hand over territory you already own to yourself, obviously).
-Additionally, you cannot ask a country several times, so if all the nations that satisfy the above rule have already been asked you won't be able to ask them again.
To be complete, by "that possesses the involved territory" I mean at least one of the states with the following ids :
284 629 630 631 632 633 634 635 636 638 639 641 642 643 645 646 647 648 684 723 725 726 727 734
I'm not gonna look them up one by one but here's a map for state ids that should be at least valid until 1.9.3 :
https://hoi4.paradoxwikis.com/images/b/bb/State_ID_map.png
Additionally, to be able to SEE the decisions at all, you need :
-To be New Zeland or a successor state of New Zealand.
-Not to have already formed Polynesia
-To have completed the focus Maori Volunteers (yeah, it's both in the "available" and "visible" conditions)
-And to be in a faction with blah blah the same thing as above.
If you satisfy all these conditions you should be able to see AND take the decisions to demand the territories. If I didn't read the files wrong.
