The horses were in Tasmania in January last year to compete in the Barnbougle Polo event.

However, on their return to the mainland, it was discovered they had died after being transported on the Spirit of Tasmania.

As a result of a year-long investigation, the Department of Primary Industries, Parks, Water and Environment has laid charges at the feet of TT-Line along with two drivers of the horse transport company.

The charges were brought against the two people and the company arose after an investigation undertaken by the Department of Primary Industries, Parks, Water and Environment.

TT-Line and the driver of the vehicle in which the horses died have been charged under section 7 of the Tasmanian Animal Welfare Act for allegedly using a method of management that was reasonably likely to cause unreasonable and unjustifiable pain and suffering to an animal or group of animals.

The driver of the second vehicle (whose horses did not die) is facing charges under regulation 34 of the Tasmanian Animal Welfare Act for allegedly failing to ensure horses being transported across the Bass Strait were individually stalled.

If found guilty, TT-Line may face a fine of up to $163,000. The drivers face either six months in jail or a $32,600 fine (or both) for offences under section 7.

For offences under section 34 corporations face a $16,300 fine and individuals may face an $8,150 fine.

TT-Line chief executive Bernard Dwyer said the company would "vigorously defend itself" against the complaints made.

"We have been working closely with Biosecurity Tasmania for many months on animal transport and are surprised by the charges," he said.

Mr Dwyer said the safety and reliability of the Spirit of Tasmania services for passengers, passenger vehicles and freight had always been, and would continue to be, of critical importance to the company.

A DPIPWE spokesman said the company noted the work done by TT-Line in the matter of animal welfare during the investigation.

"Although charges have been laid following investigation of the matter, they relate to a specific set of circumstances and the department reiterates previous advice that there is no ongoing risk for the continued movement of horses across the Bass Strait in line with regulatory standards," the spokesman said.

Mr Dwyer similarly noted the comments made by DPIPWE in relation to the safety of animal transport.

"We note the comments made by Biosecurity Tasmania immediately after the incident that the department was confident that there was not an ongoing risk for the transport of horses across the Bass Strait and that it was an isolated incident," Mr Dwyer said.

Ten of the horses belonged to Andrew Williams of Willo Polo Club and the rest were owned by his employer, Johnny Kahlbetzer, the son of agribusiness baron John Dieter Kahlbetzer.

The men and TT-Line will face court at a later date in relation to the charges and the investigation.