Victoria's biggest power providers have warned against a Coalition promise to underwrite a large new power station, arguing government intervention in the energy market could stifle private investment, with long-term consequences for customers.

On Monday, the Coalition promised to build a new power station for the state using gas or coal to meet the electricity needs of government services, including hospitals and public transport – while also shaving about $350 off the average annual household bill.

Opposition Leader Matthew Guy has promised to release a tender for a 500-megawatt power station that could be built as early as 2021.

Opposition Leader Matthew Guy. Credit:AAP Brendan McCarthy

But the owners of the state's existing coal-fired power plants cautioned against the proposed government intervention, arguing it could lead them to invest less in their existing plants, which still provide the bulk of Victoria's power.