Australian bushfires killed at least 25 people in the southern state of Victoria as a heatwave sparked more than 40 blazes across the state and neighbouring New South Wales, police said.

A spokeswoman for Victoria state police said the deaths were spread across various parts of the state.

Earlier, Victoria's deputy police commissioner Kieran Walshe said 14 people had died in the massive fire 80km north of Melbourne - six at Kinglake, four at nearby Wandong, three at Strathewen and one in Clonbinane.

The six killed in Kinglake were all in the same car.

Mr Walshe said the final death toll could reach as high as 40 as emergency services begin their search of the areas.

The Victorian bushfire had burnt some 3,000 hectares of mainly national park, before it flared in the afternoon when a cool southerly change hit with strong winds fanning the fire into local towns. Within hours the fire had burnt some 30,000 hectares, said fire officials.

Police said dozens of houses were destroyed in Victoria and local media said at least 100 homes had been burnt down in fires in the two states.

Australian fire officials have advised residents to stay and defend their homes against bushfires, as most homes are damaged not by the firefront but by embers blown onto roofs. Evacuation is a last resort and residents are told to leave early.