The locals actually call this fall Horsetail Falls, not sure why. It is indeed seasonal. My partner and I have driven past this fall a number of times, most times there is water going over it but on other occasions there is none.
The road coming out of Queenstown, named 99 bends by locals, hides the waterfall until you are right on top of it, it is an awesome sight when you turn the corner and you see a ribbon of water flowing down the mountain.
Definitely one of the nicest waterfalls, easy to access. I would recommend stopping and looking to everyone.