Sixty vehicles have broken free after a torrent of water trapped them on State Highway 6 south of Queenstown.
The NZ Transport Agency (NZTA) said the drivers were trapped between slips just north of Kingston.
But after a few hours waiting in their cars, the road was cleared and they've moved north to Queenstown.
Extensive flooding has caused major problems across the lower South Island.
Roads in northern Southland have been hammered by rain and conditions are challenging for contractors.
The Southland District Council has warned residents that crews may not be able to access all affected areas.
"Be sure to check in with [the] NZ Transport Agency before travelling through western areas of the South Island," MetService said on Facebook.
"The heaviest rain is forecast to move up Westland today [Tuesday]."
Rain has been falling at a rate of 40 millimetres per hour in Fiordland - which could mean another metre has fallen by the end of the day.
More to come.