there are plenty of practices which occur in the USA and other countries which are outlawed in Australia and NZ (amongst other countries). For example, in Australia it is illegal to debeak birds, and different animals must have a certain amount of living space.
However the only way to know is to visit the farms and the slaughterhouses yourself and see it with your own eyes. What happens there may well occur outside the law and "mistakes" happen, for example, animals not being killed in the "correct" way.
If you need proof, go and see for yourself. I believe you shouldn't eat anything unless you know where it has come from.
ETA: In Peter Singer's book which he wrote with Jim Mason (an Aussie), "The Ethics of What We Eat", they discuss Australian farming practices extensively, and I wouldn't be surprised if they were similar in NZ, but I am not sure.