More than a dozen cows have been thrown into the Karangahake River after the stock trailer they were travelling in came loose from the rest of the truck.
Waikato police Senior Sergeant Andrew O'Reilly is now urging motorists to steer clear of State Highway 2 between Waihi and Paeroa.
Northern Fire and Emergency shift manager Dallas Ramsay said emergency crews were called to the scene about 9.20am.
There were at least 10 to 15 cows that had landed in the river while others were still trapped in the trailer of the stock truck.
A vet had since arrived at the scene to help assess and possibly put some animals down.
O'Reilly said it appeared as though the trailer of the fully laden stock truck had somehow come loose and fallen into the river.
The driver of the truck was not seriously injured.
There were a lot of cows at the scene.
The road remained open but it was down to one lane, causing delays.
The road would be closed at some point as a crane would need to be brought in to retrieve the trailer from the river.
He said the crash happened on the Waihi side of the Ohinemuri River Bridge.