What is this Tunnel for?

Ben over at ben.run is intrigued by the above tunnel, located in the Ngaio gorge valley...

It looks old, however there is a pipe in it that appears to be still connected. If there is a pipe in the tunnel, then it must have another end, rather than just a borough in the hill. So what is it for? When was it built? What is it used for now? And most importantly, where is the other end??

If you've got any ideas, head over to ben.run and leave a comment.