In 2015, renewable energy generated 40.1% of New Zealand's gross energy supply. The majority of the country's electricity supply is generated from hydroelectric power, with major schemes on the Waikato, Waitaki and Clutha / Mata-Au rivers, as well as at Manapouri. Geothermal power is also a significant generator of electricity, with several large stations located across the Taupō Volcanic Zone in the North Island. The five main companies in the generation and retail market are Contact Energy, Genesis Energy, Mercury Energy, Meridian Energy, and TrustPower. State-owned Transpower operates the high-voltage transmission grids in the North and South Islands, as well as the Inter-Island HVDC link connecting the two together.