Flooding is affecting several parts of State Highway 6 between Ross and Franz Josef Glacier on the West Coast, the Transport Agency says.
The stretch of highway is still open, but motorists were told to take extra care.
South Island motorists were urged to drive carefully this weekend, with further heavy rain expected for parts of Nelson, Buller and Canterbury.
MetService has issued heavy rain warnings today for Nelson and Buller, as well as the Canterbury river headwaters and lakes, and will be in place over the weekend.
Transport Agency spokesperson Tresca Forrester said heavy rain could cause surface flooding and slips, as well as reduced visibility in the alpine passes.
She said more than 100mm of rain was expected, and roads would be very slippery, and motorists need to take care.