The 500m from a Great Walk applies to all Great Walks all year round, but the Whanganui has the additional "No hunting from the Whanganui River between 1 October to 30 April." so I wouldn't count on the 500m saving your gear (or FAL).
The wording "No hunting from the Whanganui river" makes it sound like you aren't even allowed to use the river to access other areas for hunting.
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