There are a lot of noxious plants growing in waterways around NZ. It is impossible to eradicate it all but it is still illegal to collect it.
There are 4 types of Elodea in NZ but one of them is legal - Elodea canadensis.
Same with the other, which may be a type of Myriophyllum. Some are illegal but others are not.
Not sure if the average hobbyist (or MAF employee for that matter) would be able to tell one from the other.
If you collect plants from waterways, and there is a chance it may be a noxious species, be very careful when disposing of it.