The Kaharoa eruption, the most recent rhyolitic volcanic event in New Zealand, resulted in the deposition of the compositionally distinctive Kaharoa Tephra over at least 30 000 km2 of northern and eastern North Island. The tephra forms an isochronous marker bed for various late-Holocene stratigraphic and palaeoen vironmental studies but is particularly important for evaluating the chronology of New Zealand's notably short prehistory because it was erupted within the last millennium, close to the time of first Polynesian colonization. We derive a definitive radiocarbon age, previously ambiguous, for the Kaharoa Tephra of 665615 BP using cluster analysis of 22 radiocarbon ages relevant to the Kaharoa eruptive episode. The error-weighted mean age obtained on unscreened ages, minus outliers, is supported by statistically identical ages obtained from three sets of screened ages selected to minimize the effects of inbuilt age or contamination. Based on the intercepts method and 1 sigma range of Northern Hemisphere calibration curves, and after subtraction of 27 years for the interhemispheric offset, the radiocarbon age 665615 BP corresponds to calibrated dates ranging from c. 650–560 cal. BP (i.e. cal. ad 1300–1390). The approximate midpoint of this range provides a colloquial calen dar date for the Kaharoa Tephra of c. 600 cal. BP (c. cal. ad 1350).