A non-linear soil model was established for estimating the earthquake response of the Gisbome Post Office site based on the results of geotechnical site investigation and laboratory testing on soil samples. The site was found to be very soft and susceptible to seismic amplification. The non-linear model was able to portray the response spectra of recorded motions reasonably well using modified records from a rock site with a thin layer of stiff soil. Sensitivity analysis showed that the ground surface spectra were not very sensitive to variation of the soil parameters over a plausible range. A suite of rock site records obtained from moderate and large magnitude earthquakes was used as the excitation for the non-linear model and the spectral ratios were expressed as functions of the spectral values of the bedrock excitation at the same period. The scatter of the spectral ratios was reasonably small and the cause of the scatter was found to be the different frequency content of bedrock excitations. For a very large bedrock motion, soil non-linearity caused deamplification with respect to the rock site spectra for periods up to the elastic period of the fundamental mode of the site, while spectral ratios changed little with the spectral values of bedrock excitations for periods much longer than the site period, indicating that soil non-linearity has little effect at long periods. These ratios were used to calculate a site-specific smoothed spectrum based on a probabilistic rock site spectrum for a given probability of exceedance. The smoothed spectrum is suitable for engineering design applications. Finally, a comparison was made of the results from an equivalent linear method employed in the SHAKE program and the non-linear analysis. For this very soft soil site, the difference in response spectra at short and intermediate periods by the two methods was found to be significant for moderate and large bedrock shaking.