The Murray River has peaked at the South Australian border but authorities say towns downstream should remain on high alert.

The river has been dropping at Renmark by about six gigalitres a day for the past three days, State Emergency Services chief Chris Beattie said.

“We’re confident this indicates the peak has passed,” he said on Tuesday.

“All of our gauges downstream from Renmark continue to show rises and we’ll expect to see that outlook continue for the next two-and-a-half to three weeks as the peak flows through.”

The flood levee has held up at Renmark but the downstream Mypolonga levee was breached on Monday and it could be overtopped in coming days, Mr Beattie said.

The Bolto community, about 100 kilometres east of Adelaide, is also isolated after the levee at Mannum was breached.

Authorities would continue to do welfare checks to ensure the remaining Bolto residents were safe, Mr Beattie said.

Three adults and a child had to be rescued from floodwaters on Monday after their boat failed near Blanchetown.

The group had been returning to collect furniture from their property when their vessel sank and they had to cling to trees.

“The near miss is a timely reminder of the dangers of the river,” Mr Beattie said.

“I encourage all people that have to use the river either for work or to access a property to be very mindful of the restrictions.”

All non-essential activity on the Murray has been banned, including boating, fishing and swimming. Multiple ferry services remain suspended.

More than 3000 houses and properties remain without power, with more disconnections expected.

SA Premier Peter Malinauskas on Tuesday announced $10,000 grants for small businesses, farm enterprises and not-for-profit organisations affected by the floods.

The grants will be available to those businesses along the river who have experienced a downturn of 30 per cent over a three-month period.

“We’re hopeful that all businesses will survive beyond the floods,” Mr Malinauskas said.

Meanwhile, ex-tropical cyclone Ellie is dumping intense rain on the Northern Territory’s Barkly region near Tennant Creek before moving northwest towards Western Australia.

Despite the change of direction, authorities warn that life-threatening flash flooding is possible, with 24-hour rainfall totals up to 300 millimetres and damaging winds forecast for some areas.

A severe weather warning and flood warning remains in place for various rivers and catchments in the north-western, central and southern NT.

-AAP