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By Melissa Brown
Topic:Floods
Dozens of vehicles were flooded, including these ones at Cumberland River. (ABC News)
Record-breaking rain fell on the popular holiday towns along Victoria's Great Ocean Road in the space of a few hours on Thursday, triggering flash flooding that swept away cars, caravans and tents. 
Between 200 and 400 people in low-lying areas were displaced, many of them holiday-makers in caravan parks, and dozens of vehicles were impacted by floodwaters, according to the State Control Centre (SCC).
At least six people were rescued by emergency services during the flash flooding, but no-one has been reported injured. Some foreshore areas may be closed for weeks as a significant clean-up and repair effort gets underway.
Photos from the scene this morning captured the scale of the task ahead, with cars lodged in river systems, campgrounds turned to mudflats, and people plucking their belongings from the shoreline.
But what caused such an intense rain system to unleash havoc on the coastal communities?
A number of thunderstorms developed across Victoria yesterday — most in the north and east of the state.
The weather conditions that helped them form can be traced back to ex-Cyclone Koji, which has caused flooding in some parts of outback Queensland.
Bureau of Meteorology (BOM) senior forecaster Angus Hines said Victoria was ripe for thunderstorms yesterday because of very high moisture in the atmosphere.
"When [the cyclone] arrived in the country, it brought this big swathe, this big pull of very humid conditions onto Australia, which has since been dragged down the east coast," Mr Hines said.
"Essentially, what that meant was any storms that did develop yesterday through convection or developing clouds were able to tap into all that moisture … and bring very heavy rainfall."
The clean-up along the Great Ocean Road is expected to take weeks. (ABC News)
Mr Hines said the humidity coincided with an upper-level trough, which helped intensify the stormy conditions.
He said the location of the storm in the hills above the Great Ocean Road also played a part.
"When you get this very humid air that arrives at hills or mountains or ranges, it's got to lift, it's got to rise to go over the terrain," he said.
"That upwards movement — that lifting pattern — essentially just causes more of the humidity in the atmosphere to condense into water, into little droplets, and those droplets go on to create clouds, and those clouds go on to create rainfall.
"It led to intense rainfall in and around that Lorne area, and that very heavy rain falling through the day had a very swift and very severe flood response."
Potentially contaminated floodwaters surged out into the ocean. (ABC News)
The heavy rainfall in the hills happened in a catchment area that emptied out along the Great Ocean Road, an area popular with tourists at this time of year.
Flash flooding occurred in several communities along a long stretch of the coast, including Lorne, Separation Creek, Cumberland River, Wye River and Kennett River.
Because it's the summer school holidays, many caravan and camping sites were packed.
Damage assessments were continuing today, but a number of cars were washed away and others were inundated along with caravans and tents.
Many holiday-makers lost the possessions they had with them. It's likely some were caught off guard, perhaps at the beach or away from their campsites, before the downpour was unleashed.
"Flooding has occurred in such a rapid point of time and, unfortunately, some people have been unable to relocate those vehicles," Alistair Drayton from the Victorian State Emergency Service said.
The floodwater swept through caravan parks, rising in minutes. (ABC News)
Summer holidays are one of the most significant periods for tourism operators and local businesses.
Some tourist facilities, such as the Lorne Foreshore Caravan Park, could be closed for weeks as they cleaned up, repaired damage and replaced electrical items.
Mr Drayton said another issue was the significant amount of mud throughout the affected communities.
"That level of deluge would flush any creeks or river systems out, so it would collect all the debris built up over years and years and years and years," he said.
"It would have had its biggest surge and been flushed yesterday."
He said Department of Transport teams would have to inspect a number of bridges and roadways that were hit by vehicles and debris.
Cars were still floating in the ocean on Friday. (ABC News)
One of the bitter ironies of the disaster is that some communities now dealing with record rainfall are less than 100 kilometres from bushfires that have been challenging crews in the Otways.
As countless warnings were issued for bushfires across the state, the BOM issued a severe storm warning for the risk of heavy rain and flash flooding at 11:47am on Thursday.
This was escalated to a warning for potentially intense rainfall and life-threatening flash flooding at 12:40pm.
"One important thing to note for this particular area and this event is that the Otway Coast basin is what we call a flash flood catchment area, so rivers like the Wye and the Cumberland and also Separation Creek are flash flood areas," the bureau's Diana Eadie said on Thursday.
VicEmergency at 12:58pm on Thursday declared a Watch and Act warning, urging people in the Otways and inland from Lorne to take shelter due to the thunderstorm.
It warned that intense rainfall could lead to life-threatening flash flooding.
An hour later, another Watch and Act was issued for numerous communities along the coast, including Lorne, Separation Creek, Skenes Creek and Wye River.
"This thunderstorm is very dangerous," it read.
It was upgraded to an Emergency warning at 2:36pm, with emergency broadcasters asked to break into programming with the Standard Emergency Warning Signal used for cyclone and bushfire emergency messages.
The warning said "very dangerous" flash flooding was occurring at Wye River, Kennett River, Cumberland River, Lorne and surrounding areas.
Inspectors will need to check the integrity of bridges along the Great Ocean Road. (ABC News)
These warnings were issued on the VicEmergency app and website.
The State Control Centre said 10,000 people in areas along the Great Ocean Road also received emergency texts alerting them to the flooding.
Emergency Management Commissioner Tim Wiebusch said the first Triple Zero (000) call was made at 1:08pm for flooding at a caravan park. Another call about cars being washed away followed at 1:20pm.
"We do believe the warnings were adequate for what was needed at that point in time, and were as timely as you can be when you have that dynamic situation that thunderstorms bring," Mr Wiebusch said when asked whether the warnings could have been escalated sooner.
The Emergency warning was not downgraded to a Watch and Act until 8:22pm.
Authorities at that stage said it was too dangerous to return to the Great Ocean Road from Kennett River to Lorne.
The threat was declared to have passed at midnight.
"This is something that we've not seen before," Mr Drayton said.
"Nor has the Bureau of Meteorology, [which is] reporting the incredible rainfall that has fallen.
Holiday-makers and staff at caravan parks in the affected area have spoken about the floodwaters rising within 10 minutes.
Authorities say between 200 and 400 people in low-lying areas were displaced during the flooding.
At least one person was winched to safety from the Cumberland River Caravan Park by a police helicopter.
But there have not been any reports of lives lost or any injuries.
Forest Fire Management Victoria's Chris Hardman was at the State Control Centre this morning, describing the rapid flooding as extraordinary.
"With such a dramatic and extreme event occurring over such a short period of time, for folks to be able to get out of the way of those floods and for there not to be any missing people or people injured even … everybody is just eternally grateful," he said.
The Lorne caravan park could be closed for weeks after the flooding. (ABC News)
Surf Coast Shire councillor Leon Walker said the volume of water that came through was astounding.
He said it was lucky the flooding happened during the day instead of sweeping through caravan and campsites at night.
"The main thing — everyone is safe," he said.
He said his car was still packed with his own possessions from the earlier extreme heat and bushfires.
"It's very ironic that you've got a car full of fire emergency stuff and then we got flooding happening," he said.
BOM's Angus Hines has confirmed the storms above the Great Ocean Road broke daily rainfall totals for the area.
"Mount Cowley reported 180mm through the course of six hours, which is incredibly heavy rain for that part of the country," he said.
"In fact, that's the wettest day that we have on record for that particular weather station."
Mr Drayton from the SES said the previous record for that area was 123mm in a 24-hour period.
After being downgraded to Watch and Act alerts at midnight, the remaining warnings were further downgraded to Advice level in the early hours of Friday.
Authorities warn there could still be dangerous hazards in the area, including floodwater, mud, debris, damaged roads and fallen trees.
Up to 15mm of rain could fall along the affected areas on Friday, creating the potential for landslips and pushing more debris across roads.
Repairs are underway in Lorne after the flash flooding. (ABC News)
The same weather system is now moving into Gippsland in eastern Victoria.
BOM says frequent showers could bring up to 30mm of rain from the Latrobe Valley through to Sale, Bairnsdale, Orbost and Mallacoota up to New South Wales.
"I'm not suggesting for a moment that we're going to be seeing the type of outcome that's occurred over there," Mr Hines said.
"There's an incredible amount of moisture in the air at the moment, so I would urge everyone to take heed of that."
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