A persistent swarm of earthquakes occurs 25 km southeast of the summit of Mt Ruapehu, an active volcano in the central North Island of New Zealand. 319 swarm events recorded during a six-month seismograph deployment in 2001 have been relocated using the double-difference technique. The relocated events define a vertical sheet in the mid crust, with most activity 11--21 km deep. Both focal mechanisms and a stress inversion indicate strike-slip motion for the swarm events, in contrast to the normal faulting seen on a parallel active fault mapped at the surface. Various lines of evidence suggest the swarm is of tectonic origin and due to fluid movement. It currently reflects the regional stress field rather than local stresses related to volcanic activity at Mt Ruapehu. Nevertheless, changes in the activity of the swarm appear to correlate with eruptions, and the swarm appears to have potential as a stress meter for monitoring nearby volcanic activity.