The closure of State Highway Six means about 50 dairy farmers won't be able to get their milk to the Westland Milk Products factory in Hokitika.
The Transport Agency says State Highway Six north of the West Coast settlement of Harihari could be closed for at least two days after flood waters washed away a one-lane bridge over the Wanganui River.
A spokesperson says a temporary bridge will be transported from Christchurch to restore access if it is needed.
Westland Milk Products general manager Bernard May says milk may have to be dumped if it takes too long to fix the bridge at Harihari.
Mr May says it will be between one and three days before tankers can get back into the area and most farmers will have run out of holding capacity by Thursday evening.
He says milk should be drained down farmers' normal effluent disposal systems to avoid pollution of waterways.