Nearly all outback roads are closed across South Australia because of the widespread drenching in recent days.

Thunderstorms have affected a wide area as ex-Tropical Cyclone Yasi moves across inland regions.

Many cattle stations including Anna Creek are accessible only by helicopter.

William Creek resident Trevor Wright has flown over the outback and says an inland sea is forming.

"I think this will continue for a few days given that all of that water up near Ernabella that's coming down the creek system," he said.

"So there'll be a continuous flow there and the offset of that will be that the road system will be put into disrepair so they will need to have a contingency plan of how we're going to handle that."

The landscape is sodden across a wide area of South Australia. Yongala received 128 millimetres of rain in 24 hours, beating a record set 121 years ago.

Hallett in the mid-north also had a record fall of 98 millimetres, beating a mark set in 1890.