The cost to affected farmers in the aftermath of the Rangitata River flooding could run into millions of dollars, Federated Farmers says.

South Canterbury dairy chairman Ad Hendriks said while it was too early to know just how much damage had been done, once the water receded he expected those affected would have a lot of work to do.

"I cannot guess how much the damage from the flood will cost farmers. It will depend on how much damage has been done to pastures.

"The costs of this could even run into several millions per farm," Hendriks said.

Hendriks said once the pastures had dried up, repairs to damage equipment and property would begin.

"We're hoping to get much of the irrigation infrastructure back up and running in a couple of weeks."

Hendricks said a 700 metre irrigator could cost as much as $500,000, but many of those damaged by the flood could be repaired.

"If it has local damage, it can be repaired with pipes. The one that was damaged at Rangitata Dairy looks like it will need total replacement."

Hendriks said farmers affected faced busy a period of repairs and recovery.

"Once the water recedes there'll be some sort of coordinated help to see how quickly we can get these farms back up and running.

"They have to put up fences and quite a few people have already offered help."

Federated Farmers South Canterbury president Jason Grant said about 50 families across 17 farms had been displaced during the deluge.

"It's a very stressful time and we're just working through things," Grant said.

"The people that needed to stay in to look after their livestock did stay."

Some of the evacuated people were housed on farms that had spare accommodation, he said.

"Most of the families were taken away and some houses were damaged so those people would have had to evacuate anyway. It was inconvenient but people's safety was our main priority.

"We're not thinking too much about the future at the moment. It's just one day at a time. We're doing what we can at the moment."

Hendricks said trucks were unable to access only four South Canterbury dairy farms to collect milk.

"Fonterra have been good at getting to the farmers who need milk picked up. They are also offering help, but they have to wait until the water goes."

Fonterra Clandeboye and Studholme operations manager Steve McKnight said the Fonterra team was assessing damage caused by the flood and would develop a plan to support farmers with clean up and repairs.

"We're asking any of our farmers who need help or support during this stressful time to call our Farm Source Service Centre or Rural Support Trust."

Hendricks said the main knock-on effect of this disaster would be a shortage of feed for cattle, which would be dependant on how quickly the grass would grow back.

"​Feeding the animals is going to bring fresh problems. Hopefully it is just a short period of time as the cows will have to be milked all the way."

"If the water has been sitting, it's obviously unable to feed the cows," he said.

Grant said a phone line was started on Thursday for farmers who do not have enough feed.

"Any farmer who has surplus feed to donate can do so through 0800 327 646 or the Federated Farmers website," he said.

Hendricks said no animals were reported missing or dead in South Canterbury.

"Some animals had to be shifted with the help of a helicopter but all animals are on higher ground and we managed close to cow sheds on dry land."

The Livestock Improvement Corporation (LIC) was forced to use a Cessna aeroplane to deliver around 3300 straws of bull semen to cattle farmers in the flood affected areas at the weekend.

LIC's national artificial breeding manager David Hale said it was important to deliver semen to all farms on schedule, regardless of weather conditions.

"The dairy cow cycle takes roughly between 18-24 days. If insemination doesn't occur at the right time during that cycle, it means a three week wait until the cow is in heat again.

"This three week loss can have a number of negative flow-on effects for farmers, including the potential loss of three weeks' worth of milk supply and an impact on calving periods the following year."

Hale said the critical spring mating period is under way on most of the country's dairy farms, but heavy rain, slips and floodwaters had closed key roads in the South Island, making it difficult to reach a number of flood-hit farms and get the cows in-calf.

"An additional 500 semen straws have been delivered via helicopter to affected farms on the West Coast."