Lismore farmer Mr Weir helplessly watched a quarter of his 300-strong herd were swept away.

"I watched about 70 or 80 go that morning, the gate was open and there was nothing I could do," he told AAP.

An emotional Mr Weir described the moment on Monday as "one of the worst experiences I've ever had".

"To see your life's work, the genetics of your herd - helpless, I couldn't get to them, and just had to sit there and watch."

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Since then, Mr Weir has been searching for his lost cows and managed to locate about 15 of the total of 150 animals washed away, but the majority are gone.

"It's about trying to get as many of them back as I can, when you see them it's a great feeling," said Mr Weir.

Late last week, he learnt another of his cows had been found eight kilometres away, alive.

"We've fed these cows from day two of their life, and they're part of the family ... these animals are no different to your dog, it's only we've got hundreds of them," he said.

Since then Mr Weir has been overwhelmed by the help he's received. His remaining herd is being milked at a nearby property, while strangers have turned up unannounced with food.

"Those little moments of a day, that actually pick you up, it's very humbling," he said.

"I'm just overwhelmed at the support ... my phone just rings constantly."

Mr Weir's dairy farm is one of 15 in the Lismore region to suffer "catastrophic" damage due to the floods, according to the vice-chair of dairy advocacy group, eastAus Milk, Graham Forbes.

Mr Forbes, a dairy farmer from Gloucester in NSW, is helping to co-ordinate recovery efforts for farmers in need.

"We've organised helicopters to take supplies in to some of the farms, they've needed generators and electric motors to get their dairies going again," Mr Forbes told AAP.

Getting cows milked and fed, as well as removing the carcasses of dead animals, are the main priorities.

"There are dead cows sitting around on some people's lawns in Lismore, they've got to be disposed of," he said.

eastAus Milk CEO Shaughn Morgan said it will be years before the affected dairies are back on their feet.

"Farmers are resilient, they will get back on to their farms and they will want to restore production to provide food security for Australians," he said.

Because NSW and Queensland provide the bulk of fresh domestic milk on the east coast, "this will have an impact upon milk production."

NSW Farmers dairy chair Colin Thompson is urging farmers to reach out for help, noting the NSW government has set up a state co-ordination centre to assist those affected.

"Farmers affected by the floods should complete the NSW Department of Primary Industries natural disaster damage survey ... so the government can get a good idea of what's needed and where," he said.

- with AAP