Does anyone know for sure how NZ Immi know how long you have worked for if you are on a WHV and are only allowed to work for 12months?

I would assume it's by your Tax records but do they check them and contact you when you've gone over the 12months?

Has anyone been through this. I cant for the life of me remember if I've asked this question before so if I have I'm sorry just loosing the plot a bit today..... :P