Pack away your bathers and get your umbrellas ready – summer is set to be rung in with a fierce rainstorm.

As Melbourne breaks records on Wednesday for warm spring weather, we are being asked to brace ourselves for "extreme" rain to hit on Friday.

The city is forecast to be lashed with more than its monthly December rainfall average in just the first two days of summer.

Meanwhile, some areas of the state are expected to cop up to 250 millimetres.