The TopNet hydrological model was calibrated based on a combination of Virtual Climate Network stations and existing sub-daily precipitation information located within Lake Waahi surface water catchment. The hydrological models were calibrated at one continuous monitoring streamflow station in each of the surface water catchments discharging to the lakes. Due to the large potential impact of water consented activities above Awaroa at Sansons Br (within Lake Waahi surface water catchment), calibration for the Lake Waahi model was carried out taking into account only winter flows. Analysis of the calibration indicates that the models are able to better represent low flow conditions than high flow conditions when compared with streamflow observations. This result is expected due to the low density of observed precipitation gauges across the catchments and the area of the surface water catchments. Validation of the models over the whole period of record indicates that the hydrological models reproduce the range of hydrological characteristics observed at the gauging stations. Over the period 1973-2013 Lake Whangape inflows are estimated to average 5.81 cumecs (ranging from 3.80-8.8 cumecs) while Lake Waahi inflows are estimated to average 1.88 cumecs (ranging from 1.00-2.57 cumecs). Inter-comparison of the calibrated Topnet parameters used for both catchments indicates that most of the parameter multipliers are similar across both models. This indicates a consistency of the hydrological process representation across the large watershed. The TopNet hydrological model is routinely used for hydrological modelling applications in New Zealand. It is a spatially distributed, time-stepping model of water balance. It is driven by time series of precipitation and temperature data, and of additional weather elements where available. TopNet simulates water storage in the snowpack, plant canopy, rooting zone, shallow subsurface, lakes and rivers. It produces time series of modelled river flow (under natural conditions) throughout the modelled river network, as well as evaporation. TopNet has two major components, namely a basin module and a flow routing module. Climate information was obtained from Virtual Climate Station Network. Annual average precipitation and evaporation were estimated by NIWA. Spatial information in TopNet is provided by national datasets on catchment topography (i.e. 30m digital elevation model), physical (Land Cover Database, Land Resource Inventory, and hydrological properties (River Environment Classification). The method for deriving TopNet initial parameter estimates from GIS data sources in New Zealand is given in Table 1 of Calrk et al. (2008). [Clark, M.P.;Rupp,D.E.;Woods,R.A.;Zheng,X.;Ibbitt,R.P.;Slater,A.G.;Schmidt,J.;Uddstrom,M.J.(2008). Hydrological data assimilation with the ensemble Kalman filter: Use of streamflow observations to update states in a distributed hydrological model. Advances in Water Resources 31(10):1309-1324.]