There are courses and trainings to support you with teaching NZSL. NZSLTA organises a national NZSL teaching conference biennially and local workshops are given once in a while. If there isn't one in your area, attend one in another area.
Victoria University of Wellington offers the country's only course for sign language teachers: Certificate in Deaf Studies: Teaching NZSL. Tertiary institutions around New Zealand also offer a range of courses in teaching school children and second language teaching to adults e.g. BA in Teaching, MA in Applied Linguistics.
These NZSL teachers will tell you how much they benefited from training.