Queens Highway is closed near Semo Village in Sigatoka as the road is badly damaged and has partially collapsed.
People travelling to Nadi or to Suva are stranded on both sides.
The Fiji Roads Authority says a significant road drop-out has occurred along the Amro Crossing resulting in the collapse of the carriageway, cutting off traffic in both directions and making the route impassable.
They say an immediate assessment will be carried out once the floodwaters have receded.
In the meantime, drivers are strongly advised to avoid the area until further notice, as FRA works to establish a temporary detour to restore access.
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