In reply to r.king:
Is it just for NZ?
I wouldn't say this for any other country, but you may be able to get away without it. ACC here means any accidents in NZ are fully covered for health costs, probably the only reason the adventure sports industry is viable. If you're a Brit, AFAIK, urgent medical problems are covered under reciprocal agreement with the NHS. This just leaves non-urgent medical problems and dental uncovered, maybe a slight gamble. I don't think mountain rescue costs anything, but just confirm this fit non-nationals.
If you want baggage cover etc, you could then get a cheaper travel insurance policy.