Steady rain has started to fall in some outer Melbourne suburbs such as Coldstream, which has had 17mm since 9am on Wednesday, most of which fell overnight.

The highest rainfall totals overnight were in the Strathbogie Ranges in the state's north-east, which had falls of between 35-45mm, and Wangaratta, which saw 37mm of rain.

"The troughs are developing from the west, but with individual showers and storms coming from the north," said Bureau of Meteorology forecaster Dean Stewart.

"We are still looking at falls between 15-50mm, most likely around Melbourne, but depending on what parts get storms they could get more than that."

The average December rainfall for Melbourne is just under 60mm.

Some parts of Victoria could see rainfall totals of 30 to 50mm, and local totals could reach 50 to 100mm.

"Some suburbs could get their average December rainfall," said Mr Stewart. "It will be heavy enough to cause flash flooding around Melbourne."

The State Emergency Service received 29 calls for assistance overnight, mostly for fallen trees. They also issued warnings to Victorians who live or work along rivers or streams to monitor forecasts, and move to higher ground if flooding should develop.

"We are expecting much more rain today and tomorrow, we urge everyone to prepare their properties and to drive to conditions - never, ever drive into floodwater. It only takes 15cm to float a small car," said an SES spokeswoman.

The flood warning warns catchments are relatively dry, and waterways including the Yarra River, Maribyrnong River and Werribee River were expected to rise.

The Bureau of Meteorology has warned of the potential for severe weather throughout eastern Australia this week due to Tropical Cyclone Owen, which is sitting over the Gulf of Carpentaria.

The next few days will be an "extreme weather week", Senior Weatherzone meteorologist Brett Dutschke said on Wednesday.

The "zombie cyclone" started last week near the Solomon Islands, and reached category 1 on Monday. It was then downgraded before it re-strengthened to its current category 1 status. It is forecast to become a category 3 cyclone by Thursday morning.

James Taylor, manager of the bureau's extreme weather desk, said a low-pressure system developing over Victoria "has the potential to draw moisture southward from Tropical Cyclone Owen, creating a large cloud band with associated severe thunderstorms".

"Heavy rainfall associated with the cloud band looks likely to affect Victoria on Thursday then contract southwards during Friday."

Rain and thunderstorms are expected to develop across the area from the west during Thursday morning.

The State Emergency Service has warned people not to drive through floods.

"Be alert that in areas recently affected by fires, heavy rainfall increases the potential for landslides and debris across roads," the SES warning reads.

With Laura Chung