I feel your pain, really the only way to choose a tradesperson is word of mouth I have found.
I have just had some trades people in today to do some work, they weren't who I choose as it was an insurance repair job so didn't have a choice. They rushed the job which means they are going to have to come back to redo part of it. Worst of all broke something and hid it in the cupboard and didn't let us know, hoping I wouldn't notice. Luckily I took photos before they started the job, so I have proof it wasn't damaged before they came. Pays to always take photos before you have trades people in.
I don't know what has happened to the quality of tradespeople in NZ.