Thunderstorms sweeping across the bushfire-ravaged Australian state of Victoria dampened some of the deadly fires with patchy rain, but lightning sparked a new blaze and threatens to wreak more havoc in the coming days.

Hundreds of people have evacuated their homes as four large blazes burned out of control and stretched fire-fighting resources. Three of the fires are less than 100 kilometres from Australia's second-largest city, Melbourne.

Country Fire Authority deputy chief officer Graham Fountain said light rain in some areas brought some relief to hundreds of weary fire-fighters and people who had stayed to protect their homes from the burning embers raining down on them.

"However, unfortunately it wasn't enough to assist with the extinguishment of the fires totally," Mr Fountain told Reuters, adding that lightning sparked a fire 70 kilometres northwest of Victoria's biggest blaze in the Grampians National Park.

The Grampians fire has burnt through more than 120,000 hectares of land, an area the size of Hong Kong, and has a fire front of 350 kilometres. It has razed 24 homes and killed 61,000 sheep.

"There is concern that there could be more lightning with the storms as they progress over the next couple of days. But hopefully we will have enough rain to bring some relief and some extinguishment to some of these major fires," Mr Fountain said.

The Bureau of Meteorology issued a new severe thunderstorm warning yesterday afternoon.

"The critical time is now on us today, tomorrow and the next day before we expect some cool change to occur across Victoria," Victorian state Premier Steve Bracks told reporters.

The Victoria blazes have killed three people, a father and son whose car was engulfed in flames after they crashed while racing to save a family home and a fire-fighter who was killed when his tanker rolled during mopping-up operations.

Mr Fountain said that while the light rain and a small drop in temperatures from the peaks above 40 Celsius earlier in the week had eased the bushfire threat to many towns and communities, people should not become complacent.

Earlier this month, several homes north of Sydney in New South Wales and in western Victoria were destroyed by bushfires.

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