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Hedychium gardnerianum
Appears to be some kind of lily. Leaves tend to splay along one axis, making the plant look kind of flat. Grows in clusters, almost like rafts.
Apparently this is classed as an invasive species in places like New Zealand and can be very difficult to keep under control or get rid of.
Thanks for the suggestion, @LydiaMonroe! You got me on the right track. I believe it's a Ginger Lily (Hedychium gardnerianum)