The remaining fragments of the once thriving racing empire owned by former mining magnate Nathan Tinkler will be sold off next month.

In what has been described as Australia's largest ever private dispersal sale of thoroughbreds, Tinkler's racing assets, including Patinack Farm properties in the Hunter Valley in NSW and south-east Queensland as well as more than 550 horses, will go under the hammer.

The sale, to be held by Magic Millions, will be in the last week of September.

Reports estimate Tinkler could owe Magic Millions' owner Gerry Harvey up to $40 million.

"This sale is the largest unreserved, private thoroughbred dispersal sale in Australian history," Magic Millions' managing director Vin Cox said.

Patinack Farm employed up to 200 people as Tinkler became one of Australian racing's biggest spenders.

Once one of the country's youngest billionaires, Tinkler first encountered financial difficulties in April 2012 with his estimated worth dropping from almost $2 billion to $600 million because of falling coal prices.

And since embarking on a spending spree at yearling sales in Australia and New Zealand in 2008, Tinkler's thoroughbred interests have been gradually reduced.

He was forced to sell All Too Hard, a Group One-winning half-brother to Black Caviar, for a reported $25 million to Vinery Stud and his current racing team is a skeleton of its former self.

In May it was announced Tinkler had struck a deal to sell Patinack Farm to a Dubai-based firm.

But the sale, tipped to net Tinkler $130 million, didn't eventuate as doubts emerged about the bona fides of the buyer Cibola Capital.

His former private trainer, Sydney-based John Thompson, said the small number of Patinack Farm horses which remained on his books would be part of next month's sale.

"It's been coming for a while and it's time to move on. There's not much more I can say," Thompson said.

The accomplished stayer Tremec, a horse expected to figure in some of the significant Cups races during the spring, will be one of the Thompson-trained horses to go under the hammer.

High-profile stallions Murtajill and Lope De Vega as well as eight Group One-winning broodmares will also be catalogued.