From the INZ Instructions:
http://www.immigration.govt.nz/opsmanual/index.htm
b.When determining whether a medical waiver should be granted, an immigration officer must consider the circumstances of the applicant to decide whether they are compelling enough to justify allowing entry to, and/or a stay in New Zealand.
c.Factors that officers may take into account in making their decision include, but are not limited to, the following: i.the objectives of Health instructions (see A4.1) and the objectives of the category or instructions under which the application has been made;
ii.the degree to which the applicant would impose significant costs and/or demands on New Zealand's health or education services;
iii.whether the applicant has immediate family lawfully and permanently resident in New Zealand and the circumstances and duration of that residence;
iv.whether the applicant's potential contribution to New Zealand will be significant;