VICTORIA is bracing for thunderstorms and heavy rain expected to lash the state during the next 24 hours.

Bureau of Meteorology forecaster Andrea Peace said 20mm to 50mm of rain was expected to fall across Victoria but isolated falls could reach 75mm in some parts.

The bureau issued a weather warning for severe thunderstorms predicted to cause damaging winds, very heavy rainfall, flash flooding and large hailstones this afternoon.

The warning relates to the northeast, East Gippsland, West and South Gippsland, Alpine and parts of the northern country and north central districts.

The towns of Shepparton, Wodonga, Wangaratta, Traralgon, Bairnsdale and Orbost are likely to be affected.

Wimmera and parts of the Mallee, northern country and western districts are expected to experience heavy rainfall and flash flooding.

Ms Peace said the wet weather was a combination of moisture from ex Cyclone Olga, which has hovered over NSW for the past week, dragging down into Victoria, and a trough moving across from the state's west.

The State Emergency Service has urged people to take care when driving on wet roads and to clean out gutters and storm water drains around the home.

Meanwhile, Victoria's environmental watchdog has warned beachgoers not to swim at more than 30 beaches around Port Phillip Bay tomorrow,  from Queenscliff on the western side to the Mornington Peninsula on the eastern side, as stormwater is expected to affect water quality.