Heavy rains on Thursday sent a deluge of water flowing through some parts of the eastern state of Victoria, which is still battling 10 major bushfires.

Photo and video images in local media showed one car rolling around in muddy waters as it was carried along the Wye River southwest of Melbourne.

Two cars were still partially submerged under the ocean waves on Friday, a few steps from the sandy coastline.

A local record of 186 millimetres (more than seven inches) fell in the 24 hours to Friday morning in one spot, the state meteorology service said.

Most of that water fell on Thursday.

"We've seen severe-to-extreme heat wave, catastrophic bushfires and now extreme flash flooding in the southwest of the state," Emergency Management Commissioner Tim Wiebusch told reporters.

"It really is a timely reminder that communities need to be prepared for all types of emergencies."

The flash flood cut off the state's Great Ocean Road and forced up to 300 people to flee, officials said. Many of them were reportedly holidaying at caravan camps. One child was injured and airlifted to hospital.

"It was raining, we were all in the tent playing cards with the gang, then we heard shouting," camper Matthew Stanhope told The Age newspaper, saying he and his friends ran to a nearby hill.

"It was lightning fast and quiet too. There's no noise, just all of a sudden the water is up."

Flood risks have since eased in the state.

Victoria declared a state of disaster on January 10 after days of battling bushfires that have razed homes and killed one person north of Melbourne.

Emergency services said the fires have so far destroyed 289 homes and damaged another 18, as well as hundreds of outbuildings.