Recently Gee et al (2003) proposed a fluxmeter for the measurement of water and solute flux. Here we present the measurement of flux-concentrations of nitrate and ammonium that leach out of the rootzone of a weathered volcanic ash with these fluxmeters. The location of our experimental site was the raised coral atoll of Tongatapu, Kingdom of Tonga located in the South Pacific Ocean. Mono-cropping for economical export has been intensifying over the last 15 years and has resulted in a 10-fold increase in fertiliser imports. Concerns of the contamination of groundwater resources and internal lagoon of the atoll were expressed. We used modelling of flux concentrations to explain our measurements. The simple model is based on the equation of Steenhuis et al. (1994) and takes the preferential flow phenomena observed in these soils into account and includes a temperature dependent effect on nitrogen mineralisation. There was a good agreement between the measured and modelled nitrate concentration flux at 1 m. A large fraction of the intense tropical rains drained through the soil. Average leaching concentration ranged between 30 to 70 ppm N-NO3 and 1 to 6 ppm N-NH4. Flux concentrations were approximately twice as high as measured resident concentrations. These results indicate the importance of preferential flow in these micro-aggregated soils and should aid in estimating the nitrogen load on Tongatapu's ecosystem.