South Australia's State Emergency Service (SES) has issued a flood emergency warning for the River Murray as water flow into the state has reached the major flood level.

It says flows of between 190 and 220 gigalitres of water a day have reached the border with NSW and Victoria — the rate predicted for the peak this week.

The emergency warning area is for the river from the border to Lake Alexandrina, and the SES says people there should be prepared and consider leaving. 

SES chief officer Chris Beattie said there was a chance properties could become flooded or isolated regardless of their ability to withstand flood impacts.

"You may not be able to get out to restock your supplies.

"Floodwater may remain in the area for many weeks. Use local knowledge to assess your own risk and stay aware of your surroundings.

"You should now consider leaving. If the situation worsens, it may not be safe to go or your roads may be underwater. It may be dangerous for emergency services to assist you."

The state government on Thursday said the peak of flooding was expected "in the coming days", including possibly on Christmas Day or Boxing Day in Renmark, the largest town in the Riverland and one that is protected by several levees.

Projections show flows could drop to under 150 gigalitres per day in mid-January, and under 100 gigalitres by February.

So far 1,100 properties have been inundated, with that number expected to reach 4,000 at the peak.

Already, 3,160 properties have had power disconnected.

More than 250 businesses and 4,000 hectares of agricultural land are also expected to be impacted at the peak.

So far, 14 levees have reportedly failed. 

Chief Public Health Officer Nicola Spurrier on Friday encouraged South Australians to avoid exposure to mosquitoes and get vaccinated against the Japanese encephalitis virus if eligible.

A man in his 80s from the Riverland has become the first person to contract the virus in the new mosquito season.