The State Emergency Service has told people in the Bolto shack community near Mannum, east of Adelaide, that it's unsafe to evacuate due to significant damage to Hunter Road, near Haythorpe Reserve.
A total road closure will be in place by the end of Christmas Day with a section of the road expected to be washed away.
"The Bolto shack community is now isolated," the SES said in an alert.Â
"If you are in this area you can no longer leave by this road."
Flood warnings remain in place along SA's upper and lower Murray as communities prepare for the flood peak.
SA Emergency Services Minister Joe Szakacs said in a Facebook post on Sunday that the high water mark would reach Renmark on either Christmas Day or Boxing day.
The flood peak is expect to reach Mannum about 14 days after that.
About 4000 properties across the length of the river are expected to be inundated and more than 1100 have been flooded so far.
An estimated 255 businesses and 4000 hectares of productive agricultural land will also be impacted.
River heights are likely to be around those experienced during the 1931 floods but still well below record levels experienced in 1956.
Earlier this week all non-essential activities including recreational boating, fishing and swimming were banned on the Murray as a safety measure.
There are exceptions to the prohibition with people requiring access to the river for work, including tourism operators or those visiting their properties able to do so.
The supply of food and other provisions will also be allowed.
Police will work to educate the community about the new rules, but those who persistently breach the ban could face a fine of up to $10,000.
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