The tourist was among a group of three who attempted to cross the causeway when rapid water swept the car away at about 1pm on Tuesday.

Acting Superintendent Brendan Muldoon said the vehicle, which had been wedged against a tree about 250 metres downstream from the causeway, had gone missing from sight.

Several witnesses were at the scene when the car began to float and moved sideways, before it was completely washed off of the causeway.

The driver got out of the "small, silver sedan" as soon as it began to float, followed shortly by one of the passengers. The third person was unable to get out.

According to witnesses, the car "rolled over a couple of times before it wedged up against a tree", Superintendent Muldoon said.

"The vehicle has since disappeared and we do hold grave concerns for that third passenger," he said.

Emergency services were unable to drive to Hugh River with a flood boat because roads were "impassable" due to floodwaters, an NT Police spokesman said. A helicopter was dispatched to survey the area.

Northern Territory police are urging people not to cross flooded causeways or enter flooded rivers.

As record rainfalls drench the region, six people who set out from Western Australia for the NT have been missing for two days are believed to be bogged, Superintendent Muldoon said.

Five adults and a baby left in two cars - believed to be a Nissan Navara and a Ford Sedan - from Kiwirrkurra on Christmas Day but never arrived at their destination of Kintore in the NT, about 180 kilometres away.

A plane sent out by police spotted two cars about 4pm and a helicopter has been sent out to find the missing people. It is not yet known whether they are inside their vehicles.

Search efforts have been held back by the extreme weather, with police unable to carry out ground searches due to flooded roads.

"We're seriously concerned for their welfare and are putting all our efforts to making sure we get out there as soon as we can,"  Superintendent Muldoon said.

The lack of phone reception in the "extremely remote" area had further complicated the search operation, he said.