Just thought I'd post a picture of the tunnels through which you travel on the Watercare Tramway in the Waitakere Ranges. They're really quite small! Most were created in the days of pick and shovel and were just big enough to get a pit pony through pulling a cart. The tram on which we sat was therefore quite small and only really one average person wide! Alongside the tram track is the pipe that feeds the water from the dam to the water purification centre. The pipe is made of cast iron and was made in the UK back in 1912. Each section weighs in at about 1 ton so you can image the work involved in cutting the tunnels and then laying the pipeline back in the early 1900s.
If you're in the area it's definitely worth a visit. The twilight sessions sound interesting as you'll see glow worms, kauri snails, weta as well as other "stuff". Here's a few more images from the Waitakere Ranges.