The Transport Agency says State Highway Six near Hari Hari on the West Coast was reopened at 6:30pm on Monday - earlier than anticipated.
The road was closed last Wednesday after heavy rain and floodwaters washed out about 40 metres of the highway leading up to the Wanganui Bridge.
The agency says work to fill in the gap went better than expected and the bridge has no structural problems.
The Wanganui River broke its banks on Wednesday, washing away the road leading up to a bridge on State Highway 6.
Heavy rain and flood waters eroded more than 40 metres of the South Island highway on the northern side of the bridge, forcing travellers to make long detours.
New Zealand Transport Agency diverted the river back into its original channel and reconstructed the roadbed on Monday.
The agency's West Coast spokesperson Mark Pinner says it is unusual for the river to stay so high for so long, and that level caused problems.
He says contractors have been working long shifts to complete the work
Bernard May, from Westland Milk Products, says the company has had to double its fleet from 25 to 55 milk trucks and have been taking the long route south.
"It's a 22-hour turnaround from Harihari to go all the way down to Invercargill to unload the milk, wash the truck and to return the truck back to Harihari and change drivers."
Mr May says about 400,000 litres of milk had to be dumped - which is less than anticipated.
Tourst operators at Franz Josef and Fox glaciers say some visitors have travelled the long way round from the south, but it has been quiet.
Franz Josef businessman Marcel Fekkes says travel agents have re-routed visitors and delayed their trips to the glaciers where possible until the road is open.