The racehorse trainer Anthony Cummings, son of the ''cups king'' Bart Cummings, is fighting claims he worked horses so hard they were unable to race, took sale commissions he should not have and bought horses for a client that were not fit for competition.

In documents filed in the NSW Supreme Court, the horse breeding and training operation Patinack Farm, run by the mining tycoon Nathan Tinkler, is seeking $6.4 million from Mr Cummings and his companies, Cummings Thoroughbreds and Something Fast. It says this is the loss in value of horses Mr Cummings bought on its behalf and trained until they were broken down or lame, preventing them earning winnings.

But the claim was made only after Mr Cummings sought $173,000 from Patinack for unpaid training fees and costs.

Mr Tinkler refused to pay, saying Mr Cummings had not performed all the work he was charging for.

In a cross-claim, Patinack alleged Mr Cummings breached his duty of care when training the horses and profited at its expense when he was supposed to be acting for it at sales in 2008, where he allegedly bought more than 100 horses for the company.

Mr Cummings denies the allegations. He said he did not breach any duty of care, and any loss or damage is due to Patinack's negligence in failing to monitor the training and by letting horses be raced or trialled when it knew they should not be.

Patinack lists 15 thoroughbreds as ''horses that have broken down due to negligence'', including five that allegedly had no chance of racing.

The remaining 10 horses allegedly have a 50 per cent chance of racing, including Siderus, purchased for $2.5 million, and Metallurgical, bought for $2.2 million.

According to Patinack, Mr Cummings had a duty to buy sound horses he thought would eventually win Group 1 races and train them in a way that would not stress or injure them.

Mr Cummings denies he had a duty to prevent overtraining, denies training some of the horses and denies the horses became lame or broke down while being trained by him or his company.

Patinack also alleges Mr Cummings wrongly took commissions and fees from sales. This includes $2.8 million from $18.8 million worth of horses he allegedly bought at the 2008 Magic Millions sale, when he was supposed to be buying 58 horses for Patinack at the best possible price.

Mr Cummings says he never took a commission Patinack was not aware of, denies he had a duty to buy only horses with the potential to win and admits purchasing only some of the horses on Patinack's behalf.