The Ngatamariki geothermal field is located in the Central Taupo Volcanic Zone (CTVZ) on the North Island of New Zealand. Since 2013, it has been actively exploited as a source of heat for the generation of electricity. This process involves the extraction and reinjection of large volumes of geothermal fluid from depths of 2-3 km. Such injection processes have caused earthquakes at various locations across the globe. Worldwide, the relationship between seismicity and fluid injection is unique for each individual case and reflects differences in local geology and injection rate as well as the history of injection at each well. At Ngatamariki, cold-water stimulation tests were carried out at three injection wells prior to the commissioning of the 83MWe power plant in April 2014. Each of these tests was associated with an increase in seismicity adjacent to the injection wells. Here we present a catalog of microseismic events for Ngatamariki from 2012 through 2015 which we have created using a matched-filter correlation detection technique. After selecting the highest quality events for detection and magnitude calculation, this procedure has increased the number of earthquakes in the catalog by a factor of ~5 when compared to standard detection approaches. We also present focal mechanisms for the largest events which occurred during well stimulation treatments. Seismicity at Ngatamariki encompasses two spatial clusters, one in the north near injection wells NM08 and NM09, and one in the south near injection wells NM10 and NM06. There is little seismicity in the center of the field where extraction of geothermal brine occurs. The rate of seismicity increases markedly in response to increases in fluid injection rate during each stimulation treatment, however, this increase is most rapid at wells NM08 in the north and NM10 in the south. Focal mechanisms near well NM08 during stimulation show reverse faulting along structures oriented NW-SE while mechanisms near well NM10 during drilling and stimulation show normal faulting along structures oriented NE-SW. The dominant trend within the CTVZ is normal faulting along NE-SW oriented structures. In that context, the focal mechanisms for the events near NM10 are expected. We interpret the reverse faulting mechanisms at NM08 to represent slip along NW-SE striking structures associated with the damage zone surrounding an intrusive complex, accompanied by pressure buildup adjacent to NM08 during the stimulation treatment.