The Chasm is a spot along the Cleddau River where the watercourse disappears between a narrow chasm (as the name suggests). However, it's really more of a nature walk than it is a waterfall attraction because there's no safe way of seeing the waterfall satisfactorily. Instead, you get to look down at mostly strangely shaped rocks and potholes resulting from the Cleddau River's forceful flow as it gets channeled into the dark and narrow abyss.
For more information on this waterfall such as...
maps
directions
detailed descriptions
refer to A Guide to New Zealand Waterfalls (Story Nature Press).