We present continuous GPS observations of a surface deformation episode in the Manawatu region of the North Island, New Zealand, lasting 18 months from January 2004 to June 2005. We invert vertical and horizontal displacements of GPS sites during the event for slow slip on the Hikurangi subduction thrust. We find that up to 350 mm of slip occurred on the thrust in the transition zone between strong interseismic coupling and aseismic creep. If the entire slip in the Manawatu slow slip event had occurred instantaneously, it would have resulted in an Mw 7.0 earthquake. Given the large amount of slip, we expect that the recurrence interval for similar slow slip events may be ten years or more.