In New Zealand it is not standard practice to give a tip in any context (eg. restaurant or hiking). Wages and salaries are determined to give a fair income, and tipping is unnecessary and very unusual. Some places try to encourage tipping but please don't do it unless the service you receive is outstanding.
If the hiking trip is wholly organised overseas, follow the practice of the county of origin, unless the guides are New Zealand residents, as their wages will be set without any account given to possible tips.