High water levels flowing into Lake Albert may result in the highway becoming inundated in low-lying areas near Meningie.
A 60km/h speed limit has been imposed with plans in place for a detour along the Dukes Highway should the Princes Highway be impacted by water.
Concerns for the road grew on Thursday as the peak in river flows reached SA's Lower Lakes.
That has pushed lake water levels up, with concerns for some further flooding of local properties over the weekend when a low-pressure weather system sweeps through, increasing wave action.
With the high water mark reaching the lakes, Emergency Service Minister Joe Szakacs said that meant the river's peak had now passed all major communities along the waterway.
"While it's pleasing to see the peak edge closer to the ocean, a Flood Emergency Warning for the entire river in SA remains in place and I urge people to continue to take care," he said.
Further upstream, flows across the SA border have now fallen to about 135 gigalitres a day, down from a high of more than 190 GL.
They are expected to drop to about 60 GL a day by the end of January.
So far the bulk of the state's levee system has held up, especially those protecting major assets including one at Renmark, built near the local hospital, and one at Mannum, protecting businesses on the main street.
There have been 68 catastrophic levee failures and 168 major problems, but most of those have been with structures protecting agricultural lands.
The State Emergency Service will continue with daily inspections across the levee network paying particular attention to those at Woods Point, Monteith, Jervois and River Glen.
Mr Szakacs said some concerns still remained for the integrity of the major levees.
"By no stretch are we out of the woods yet. These levees continue to be under load," he said.
"There remains small but not insignificant risks about scarring, cracking and failing."
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