Five to 15 millimetres of rain is expected, with isolated heavier rain and embedded thunderstorms. Areas under the thunderstorms could expect up to 25 millimetres of rain.

A Bureau of Meteorology spokesman said Australia day will see much milder temperatures than the last few days.

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"We saw temperatures of up to 40 degrees today, but it will mostly be in the low to mid twenties tomorrow," the spokesman said.

"We'll see rain ease tomorrow morning for Australia day, and then maybe a cooler southwesterly wind."

A cool change has moved through the southern parts of Victoria and will be followed by a band of rain and thunderstorm activity building in South Australia.

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