Civil Defence and farmers in the Aorere River Valley in Tasman District have begun assessing the damage caused by torrential rains and massively flooded rivers.
The Aorere broke its banks on Tuesday and flooded bordering farms. At its peak, the river was flowing 11 metres above its normal level and in at least one place was a kilometre wide.
Witnesses reported seeing cows and calves floating down the river.
Federated Farmers' Golden Bay president, Graham Ball, said several farms lost much of their herds. One farmer lost 100 heifers and another lost 70 cows.
Mr Ball said power has been lost to the valley for up to two days, creating animal welfare problems for cows that cannot be milked.
The district's lines company said at least 100 farms and households were without power because of the wild weather.
Network Tasman had teams working around the region to make reconnections, but was having trouble accessing many properties because of flooding. It said it was aiming to restore the power to all its customers on Tuesday night.
At least four campsites in Tasman and Marlborough had to be evacuated because of flooding.
On Tuesday morning, campers had to leave the Onamalutu and Pine Valley camps near Havelock and the Old Macdonalds camp, near Collingwood, although some returned later in the day.
Almost 100 campers in Marlborough had to evacuate the Department of Conservation's Pelorus Bridge campsite in the early hours.
A camp manager, Craig Neal, said when the camp's flood alarm system went off at four o'clock, the campers were given just minutes to escape. He said most of the tents were left in place because people only had time to grab their bedding before driving off in their cars.
Mr Neal said the water has gone, leaving 10cm of silt and debris across the campground.