Airlines are getting strict about what you can and can't take on-board.
Personally, I'd check it in as a fragile item (well wrapped in a sturdy box) or well wrapped in a box but in your checked luggage. I've taken a lot of odd stuff like that for my daughter who has moved to NZ.
As hand-luggage, the stewards may insist you put it in the overhead compartment rather than between your feet (a flying helmet is quite a weight!) & I've seen how the hand luggage gets jammed into those overhead spaces - other passengers don't care if their heavy bag gets dumped on top of yours.
It may be worth taking a cheap helmet from home so you're not too upset if it gets lost or damaged.
Make sure your boots & other gear is clean. NZ don't like imported dirt 
Enjoy the trip, it's a wonderful country!
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Elaine
Striving to live the ordinary life in a non ordinary way
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