Thermochronological investigations of samples collected west of the Alpine Fault zone provide new insight into the early development of the Australian-Pacific (AUS-PAC) plate boundary through New Zealand that is not preserved elsewhere in the modern orogenic system of the South Island. The 40Ar/39Ar, fission track, and (U-Th)/He ages for these samples span the Cenozoic and provide direct constraints on the timing and character of two discrete episodes in the evolution of this tectonic boundary. The initial propagation of the AUS-PAC boundary through the South Island in the early Miocene is expressed as short-lived, geographically localized cooling, which we infer to be due to exhumation, beginning at 23--25 Ma. This is consistent with development of this boundary as an initially distributed zone of deformation, progressively localizing onto discrete fault structures developing primacy of strain accommodation over time. Regionally synchronous development of rapid cooling at 8 Ma corroborates the association of the present obliquely convergent character of the AUS-PAC boundary through the South Island with a marked change in Pacific plate motion at that time. Subsequent increases in cooling rate and a regionally prominent 5 Ma thermochronologic signal illuminate the progressive adaptation of the plate boundary to this new tectonic system over a period of ~3 Myr, culminating in development of the extant tectonic regime.