This paper investigates the effect of increasing the number of wind turbine generators on wholesale spot prices in the Australian National Electricity Market's (NEM), given the existing transmission grid, from 2014 to 2025. We use a sensitivity analysis to evaluate the effect of five different levels of wind power penetration on prices, ranging from Scenario A, ‘no wind’, to Scenario E that includes existing and planned wind power sufficient to meet Australia's original 2020 41TWh Large-scale Renewable Energy Target (LRET). We find divergence in prices between states and similar prices for nodes within states. This supports the Garnaut Climate Change Review assessment on the prevalence of ‘gold-plating’ the intrastate transmission network and underinvesting in interstate connectivity. We find that increasing wind power penetration decreases wholesale spot prices but that retail prices have increased in deregulated South Australia and Queensland, similarly, in Victoria. We argue that there is a pressing need to split the large generator-retail companies into separate retail and generator companies and to reassess regulatory rules more generally. Interconnector congestion limits the potential for wind power to further reduce wholesale prices across the NEM. So the need for a high capacity transmission backbone in the NEM is becoming clearer and will become pressing when Australia moves beyond its current 2020 LRET.