P crispus is an introduced plant. I think it is growing in Lake Pupuke up on the North Shore as well.
In slow or still water it grows quite thickly and can smother the water, although in flowing it tends to stay smaller as it is very fragile and the stems break easily. Hence it has established itself very fast in this stream by stems breaking and flowing downstream and growing.
Only six months ago, this stream was bulldozed and redug as it is beside a main road.