THE community's need for extreme fire warnings must be balanced with the heightened risk of arson, the Country Fire Authority says.

Telecommunications companies on Monday sent Victorians just under 5 million text messages advising of extreme weather forecasts.

A grass and scrub fire noticed about 12.30am yesterday on the foreshore at Chelsea, in Melbourne's south-east, was "definitely due to arson", CFA chief fire officer Russell Rees said.

Police are also investigating a spate of fires believed to have been deliberately lit in Melbourne's north on Friday.