The Victorian government is planning for a "very extreme" heatwave, worse than the Black Saturday weather event of 2009, with the potential to kill hundreds of people, cripple public transport and the electricity supply and strip $1 billion from the state’s economy.

As authorities brace for what could be the worst fire season in many years and with deadly blazes still burning out of control in NSW and Queensland, the state government says it is preparing for weather more terrifying than anything Victoria has experienced.

A trail of destruction at Rainbow Flat in NSW. Authorities are warning of more devastation in days to come. Credit:AAP

The Department of Environment, Land, Water and Planning's grim assessment is that a "one-in-110 years" heatwave, more destructive than the "one-in-25 years" scorchers in 2009 and 2014, is a real possibility, with the likelihood set to double in the next 11 years.

The department's planners have also warned that "severe heatwaves", which occur every two years on average, could become an annual hazard.