Four children are part of a group of seven people missing in a remote part of southern Western Australia experiencing flash flooding and heavy rainfall.
Police say two elderly male drivers, four children aged between 12 and 17 and an elderly woman left Kalgoorlie Boulder on Sunday and were driving about 650km north east towards Tjuntjuntjarra.
Western Australia Police Inspector Mick Kelly said he held urgent welfare concerns for the group, believed to be family members, as they were meant to arrive at their destination on Monday morning.
He has been told the cars have minimal food onboard but one has water, bedding and camping gear.
"We're praying that they've stayed with the car," Inspector Kelly told reporters late on Tuesday.
He said the search effort had been hampered by the conditions, as a plane looking for the group was only able to stay in the sky for about an hour on Tuesday.
"There's 120 millimetres forecast to hit our area in the next 24 hours and we've also got tropical weather forming at the north coast," Inspector Kelly said.
"But our pilots are the experts and quite simply, while we want to have every opportunity to bring these people back to the community safely, we can't afford obviously to put those that are out there looking and searching in danger."
The group is reported to be travelling in a beige Toyota Landcruiser, registration number A683 and a white Mitsubishi Triton, registration number KBC8881.
The Bureau of Meteorology has issued a severe weather warning for parts of the Goldfields, Eucla and South Interior districts because of heavy rainfall and flash flooding.
"Locally intense rainfall which may lead to dangerous and life-threatening flash flooding is also possible," the alert stated.
Anyone with information about the group's whereabouts has been asked to contact Kalgoorlie Police Station.
WA cut off as floods inundate roads, rail
Meanwhile, the sole rail line connecting Western Australia to the rest of the country is likely to remain closed for several days as parts of the state experience a year's worth of rain over a few days.
The Trans-Australian Railway line, a key freight route running between WA and South Australia through the Nullarbor, has been closed since Sunday with floodwaters inundating parts of the track.
The Eyre Highway - the main road connecting WA with SA - was also closed because of flooding but re-opened on Tuesday morning before it was closed again in both directions at 5pm.
The WA Department of Fire and Emergency Services advised those living in the Goldfields-Midlands and Midwest-Gascoyne regions to take action with severe weather on its way.
The agency also warned of flooding in the Eucla and Rawlinna areas as well as Laverton, Balladonia, Cundeelee, Ilkurlka, Warburton, Yelma and Lake Carnegie in a series of updates late on Tuesday afternoon.
More than 200mm of rain fell on parts of south-east WA in the 24 hours to 9am on Tuesday, with March rainfall records tumbling for the Eucla and Goldfields districts.
Average rainfall in those areas is about 260mm per year.
The Bureau of Meteorology has forecast further 24-hour totals of up to 150mm for affected areas during Tuesday and Wednesday.
"A near-stationary trough over southeastern Western Australia will remain in the area until the middle of the week," the bureau said on Tuesday.
"A moist air mass drawn southward from the tropics is combining with this trough to produce a band of heavy rainfall, with embedded thunderstorms capable of locally intense falls."
The system is expected to persist throughout the rest of Tuesday and into Wednesday.
Intense rainfall of up to 80mm was forecast across six hours, potentially leading to dangerous and life-threatening flash flooding.
WA's Department of Fire and Emergency Services warned residents in Rawlinna, Carnegie, Cocklebiddy, Eyre and Zanthus to seek shelter, stand clear of windows and refrain from driving through floodwaters.
"If you live in parts of the Goldfields, Eucla and South Interior districts you should take action and stay safe with severe weather to come," the department advised.
"This is not typical weather for southeastern Western Australia."
Photos posted to social media by Rawlinna Station, Australia's largest sheep station, showed the notoriously arid Nullarbor resembling an inland sea.
Given the anticipated prolonged rainfall, the east-west rail line is expected to remain closed until later in the week, the Australian Rail Track Corporation said.
The iconic Indian-Pacific train service that operates between Perth and Sydney via Adelaide has been cancelled as a result.
"Due to a severe weather incident on the Nullarbor, the Indian Pacific schedule until Saturday 16 March has been impacted," its website says.
"At this time, all departures from Sunday 17 March will operate as scheduled."
The Leonora Laverton Road remained closed to traffic in both directions between Laverton and Leonora in the Goldfields region.
Lingering monsoonal storms continued to bring heavy rain to the Top End and northern WA, with large areas receiving between 100mm to 300mm of rain in recent days.
People in Fitzroy Crossing, Noonkanbah and Mount Barnett were warned to take emergency action with minor flooding expected on Tuesday and in the coming days.
DFES personnel were deployed to assist isolated communities after the Great Northern Highway was shut in both directions between Fitzroy Crossing and Halls Creek.
More rain is on the way with the potential for tropical cyclones to develop in the Indian Ocean, Gulf of Carpentaria and Coral Sea over the next week, the BOM warned.
