You will have to apply for a working VISA. Check it our here:
http://www.immigration.govt.nz/migrant/stream/work/workingholiday/canadaworkingholidayscheme.htm
To instruct you will have need to have done instructors certificate, and then apply for a job around March to local mountains. Although it is not the best money, so you might be better to work off the mountain, and ski on you days off. (Bar/resterant work can work well with skiing). But expect to earn around $13-$15 per hour.