IRD doesn't care about cashies because
A) They cost too much to research and enforce
B) That money will likely be taxed somewhere else in the chain (for example, retail spending of the money acquired by cashies) - unlike international corporations who move all of their non taxed revenue offshore - with no chance of any of it being taxed down the chain.
I don't think that is true. The IRD has been running a campaign to track down the "black economy".
"Cash payments will always leave a trail, Inland Revenue says" https://www.stuff.co.nz/business/100078541/cash-payments-will-always-leave-a-trail-inland-revenue-says
"In its most recent annual report, Inland Revenue noted that it had found $159 million in "tax position differences" in the 2017 year." That must be a reasonable rate of return on the cost of investigation as the Govt. has allocated about another $80M