Co-magmatic granitoid clasts erupted as part of the Rotoiti Ignimbrite (Rotoehu Tephra) contain euhedral K-feldspar and biotite crystals that protrude into miarolytic cavities and show textural evidence for growth in super-cooled conditions and are thus interpreted as growing during eruption. ⁴⁰Ar/³⁹Ar stepped heating experiments on single K-feldspar crystals reveal the presence of heterogeneously distributed excess ⁴⁰Ar, preferentially released at lower temperature steps (most likely from fluid/melt inclusions), which cannot reliably be characterised by, or corrected for using isotope correlation diagrams due to mixing between three reservoirs of ⁴⁰Ar (radiogenic, atmospheric and excess). This excess ⁴⁰Ar component is common, but not ubiquitous, and an age population unmixing algorithm applied to single-crystal fusion data identifies a younger group of K-feldspar and biotite crystals that appear to be largely unaffected by excess ⁴⁰Ar. This population gives a statistically robust weighted mean age of 47.4 ± 1.5 ka (1σ, n = 13) and an indistinguishable inverse isochron age of 50 ± 3 ka for this historically difficult to date eruption. The weighted mean age is significantly younger than previous age estimates of the Rotoiti eruption obtained by K/Ar and ⁴⁰Ar/³⁹Ar dating of bracketing lavas, but is indistinguishable from recent ¹⁴C and (U-Th)/He dates and estimates based on orbital tuning and sedimentation rates constrained by ¹⁴C ages.