The magnitude of the return period of the 1998 flood peak at the Waipoua River Mikimiki hydrological recording station is a critical factor in the assessment of potential flood plain inundation in Masterton. Using rating and slope-area information available at the Mikimiki site, and assuming subcritical flow and occurrence of bed scour, the peak discharge of the 1998 flood is calculated to be 400 ±40 m3/s. From historical flood information and rainfall intensity data, the 1998 flood is deemed to be the second largest peak since 1897 which implies that it has a return period of about 75 years. This estimate combined with flood frequency analyses of site annual maxima yields a return period of 70 ±7 years for the 1998 flood peak, a figure consistent with the results of regional flood frequency analysis. The largest flood since 1897 occurred in 1947 and has a return period of about 200 years. Both the 1947and the 1998 floods were safely passed by the existing Masterton Flood Protection Scheme. The following analytical output resulted from the data: (1) Plot of apparent bed level for flow of 400 L/s at Waipoua River at Mikimiki. (2) Frequency of annual maxima (1979-1982, 1996-2014) fitted by a Gumbel distribution (1998 flood peak = 356 m3/s). (3) Stage versus velocity relationship (1996‑2014) for Waipoua river at Mikimiki. (4) Frequency of annual maxima (1979-1982, 1996-2014) fitted by a Gumbel distribution (1998 flood peak = 400 m3/s). (5) Comparison of daily rainfalls for 1947 and 1988 storms.