Personal or household income predicts mortality risk, with each additional dollar of income conferring a slightly smaller decrease in the mortality risk. Regardless of whether levels of income inequality in a society impact on mortality rates over and above this individual-level association (i.e., the 'income inequality hypothesis'), the current consensus is that narrowing income distributions will probably improve overall health status and reduce socio-economic inequalities in health. Our objective was to quantify this impact in a national population using 1.3 million 25-59-year-old respondents to the New Zealand 1996 census followed-up for mortality over 3 years. We modelled 10-40% shifts of everyone's income to the mean income (equivalent to 10-40% reductions in the Gini coefficient). The strength of the income-mortality association was modelled using rate ratios from Poisson regression of mortality on the logarithm of equivalised household income, adjusted for confounders of age, marital status, education, car access, and neighbourhood socio-economic deprivation. Overall mortality reduced by 4-13% following 10-40% shifts in everyone's income, respectively. Inequalities in mortality reduced by 12-38% following 10-40% shifts in everyone's income. Sensitivity analyses suggested that halving the strength of the income-mortality association (i.e., assuming our multivariable estimate still overestimated the causal income-mortality association) would result in 2-6% reductions in overall mortality and 6-19% reductions in inequalities in mortality in this New Zealand setting. Many commentators have noted the non-linear association of income with mortality predicts that narrowing the income distribution will both reduce overall mortality rates and reduce inequalities in mortality. Quantifying such reductions can only be done with considerable uncertainty. Nevertheless, we tentatively suggest that the gains in overall mortality will be modest (although still potentially worthwhile from a policy perspective) and the reductions in inequalities in mortality will be more substantial.