Temperatures in New South Wales have fallen more than 10C.

Scorching conditions are predicted to return at the weekend and new fires are spreading despite the temperature drop, authorities say.

The fires have destroyed buildings in New South Wales, Victoria and Tasmania.

Tasmania remains on alert with 40 bushfires burning around the state and five blazes still considered a major threat.

The New South Wales Rural Fire Service (RFS) has downgraded fire danger ratings around the state following a favourable change in conditions overnight.

Police confirmed on Tuesday that the biggest fire, on the Tasman Peninsula, had begun accidentally at Forcett.

A tree stump burn had continued to smoulder through the root system and ignited when the south of the state hit record temperatures on Friday.

The fire has burnt 23,000 hectares, destroyed more than 120 buildings and left 100 people unaccounted for.

Lightning is thought to have started an east coast bushfire that razed up to 15 properties.

In Shalvey on Sydney’s west, it was firebug teens that set fires blazing.

Deputy Police Commissioner Mick Fuller said three “juveniles” were being interviewed over the fire, which scorched 400m of grassland in furnace-like temperatures.

The premier and firefighters hailed a miracle that no lives were lost in the 132 blazes across NSW.

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