By using cash, it has the chance to get fake money coming in. i.e. photocopy the paper money. I think that I got hongkong dollar as replacement for nz dollar. I did not check that at the time I got the money. You know there is concurrency difference between hongkong and nz dollar.
I don't mind someone give me Australian dollar as replacement nz dollar. I did receive one or two Australian dollar in my 20 years living in nz. Even that, I cannot use australian dolloar in nz and it is not worth the effort to exchange the australian dolloar.
There is different rounding algorithm for supermarket to exchange money. i.e. if the price is 6.06, some supermarket will charge you 6.1 and hate that. By using credit card, you will always get same accurate charge.