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Police have "grave concerns" for seven missing people who were travelling to an Indigenous community when record rainfall hit Western Australia's Nullarbor.
The rain plunged Australia's largest sheep station underwater and closed Perth's main arterial road to the rest of the country.
Floodwaters have cut access to the Trans-Australian Railway line and the Eyre Highway, both vital freight paths in and out of Western Australia.
Both are expected to remain closed for several days.
Inspector Mick Kelly said two elderly men, one elderly woman and four children aged between 12 and 17 were driving from Boulder to Tjuntjuntjara, but had not arrived.
"Concerns are held due to the fact we have had severe weather and rain and more has been forecasted over the next 24 hours," Kelly said.
"We have heard from the community of Tjuntjuntjara that they have not arrived safely.
"We are unsure of what food and water resources they have onboard.
"We do hold grave concerns for their welfare."
The Indigenous community is 650km north-west of Kalgoorlie.
"My understanding is that they are family, they are related," Kelly said.
Police drove along the highway in an attempt to find them but it became too difficult on flooded roads.
A helicopter was deployed for an hour in the morning but the cloud was too thick to search for the families driving in a convoy.
Kelly hoped they had stayed near their vehicles which were a Toyota Landcruiser with registration A683 and a Mitsubishi Triton with registration KBC8881.
Officers will attempt another search tomorrow.
The heaviest rainfall has been seen around Eyre weather station in WA's south-east, where 325mm of rain has fallen in four days - more than the area's average rainfall for a year.
The huge downpour also decimated a century-old record for the heaviest rainfall in any single month, which was 203.8 mm in March 1912.
Just north of Eyre, Rawlinna Sheep Station is underwater.
At roughly the size of greater metropolitan Sydney, Rawlinna is the largest sheep station in the country and stocks 60,000 sheep.
Images show the normally dried-out station resembling something closer to a dry desert lake, with station hands even using kayaks to rescue the station's chickens, paddling the poultry to safety.
The rare heavy rainfall has been caused by an upper-level high pressure system centred over the Camerons Corner area at the junction of the NSW, South Australian and Queensland borders.
The high-pressure system is blocking the progression of systems from west to east, allowing moisture-laden air to flow from the tropics across the interior of WA down towards the Great Australian Bight.
It comes as the risk of tropical cyclones for northern Australia grows, with a monsoon trough hitting multiple states this week.
The Bureau of Meteorology has forecast a high chance that a tropical low currently near the Cocos (Keeling) Islands west of Western Australia could form into a cyclone from Friday.
Weatherzone reported that this system is likely to move eastward towards the mainland this week.
"If the low develops into a tropical cyclone, this will be the fifth named tropical cyclone this season," the website said.
"The low or cyclone could come close to the WA coastline later in the week into early next week."
Other tropical lows, including one far closer to the mainland just off the Kimberleys, have a lower chance of developing into a cyclone, but are still likely to have an impact.
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Weatherzone said the "active monsoon" was likely to bring "huge" rainfall across northern Australia this week.
Some areas of northern Western Australia, the Northern Territory, and Queensland, are set to receive 100mm-200mm of rain, with isolated pockets of up to 250mm.
People in northern Australia are advised to stay up to date on weather warnings.