It's not as scarey as you would think, the process is setup and works well. If you've not already the guide here -> https://www.consumer.org.nz/articles/disputes-trib... i would do so- good starting point on how to approach it.
Key test will be 'how many years ago' and what is reasonable life expectancy of the product.
Presume you are asking for a repair, given the item(s) sound old-ish- have the retailer refused to do this? If it's a chain, i would start with a letter to their head office, stating the facts and that you are seeking resolution under the act, state the steps taken thus far, or you will go to the disputes tribunal.
That's worth the price of a letter, since you can't recover the $45 tribunal fee. If this is an independent retailer, i would just be prepared to go straight to disputes tribunal if you think you've got to the Owner/Manager.