New Zealand has a clean, green image, but most streams and waterways are not safe to drink from without boiling, filtering or sterilising it first. Most of our water is safe to swim and fish in, but when the population density increases, then it is safest to check first - generally, there will be warnings if swimming or fishing is not safe.
The higher up you are, or closer to the source of the water - the safer it is, but once it has run through open territory it will have some level of contamination. This is caused by human contact, and by animals. One of the most common nasties is Giardia