Damage to the Wanganui bridge at Harihari on January 2 cut State Highway 6, meaning milk destined for Westland Milk Products could not be transported to the company's processing plant at Hokitika.
Since Thursday, 55 extra road tankers have been helping get milk to various processing plants - including 18 running continuously between the West Coast and the Open Country Dairy Ltd factory, at Awarua, Fonterra's Edendale factory and the Synlait Milk Powder Factory at Dunsandel.
Dunedin transport operator Peter Dynes said his company had answered a ''mayday'' and had a team of 36 drivers from various southern transport companies running a 24-hour milk delivery system.
Yesterday the company carried 350,000 litres but in the days when it was ''catching up'' it was shifting up to 600,000 litres in a day.
The milk was from 65 farms south of the damaged bridge.
In all, Westland Milk general manager operations Bernard May says alternative arrangements had to be made to transport 2.1 million litres of milk.
Normally, much of the company's milk is carried by rail from Canterbury but the line was also damaged in last week's flooding.
Mr May said most of the South Island dairy industry and trucking companies worked together via a crisis team when there were problems.
''It's been a very good effort by everyone in the South Island dairy industry.''
Trucking milk south involved a round trip of more than 18 hours. Drivers were stationed temporarily at Lake Hawea and Cromwell.
Mr Dynes said his company contracted exclusively to Fonterra but was given a dispensation to answer the milk mayday.
''Because of the circumstances, we rang Fonterra and they said: `yeah, get into it'.''