Part of State Highway 8 in Otago is down to one lane due to a "serious fire", the Transport Agency says.
A car has caught fire near Raes Junction, south of Roxburgh.
Fire and Emergency southern communications shift manager Brent Dunn said the fire then spread to nearby vegetation, but it has since been contained.
He said a crew would remain on sight for a while, to ensure the fire was fully extinguished.
The fire is not deemed as suspicious.
The Transport Agency said people should should consider delaying their journeys.