Spirit of Tasmania operator TT-Line should not be convicted over its role in the deaths of 16 polo ponies, lawyers have argued.
Magistrate Leanne Topfer in October found the ferry operator guilty of 29 animal welfare charges, after the horses were found dead inside a converted refrigeration trailer in January 2018.
The animals had competed at the annual Barnbougle Polo Tournament before crossing the Bass Strait from Devonport to Melbourne.
Ms Topfer found the horses were in the transport unit for about 10 hours without adequate ventilation.
There were too many horses in the trailer and 16 horses were exposed to the risk of acute heat stress and asphyxiation, and died from respiratory failure, she said.
During a hearing in Burnie Magistrates Court on Wednesday, TT-Line lawyer David Neal argued the company should not be convicted over the deaths.
He said the operator had a previously great track record, with thousands of animals safely making the same journey over the decades.
The same container had been used on nine previous occasions without issue and the horses were checked on five times throughout the January 28 voyage, Dr Neal said.
The circumstances which led to the horses' deaths were unprecedented and TT-Line could not have reasonably foreseen what ended up happening, he told the court.
Dr Neal submitted a conviction would leave a "dark stain" on the TT-Line's otherwise great record.
Former Australian polo captain Andrew Williams also asked not to be convicted after he pleaded guilty over the horses' deaths in July.
The court was told Williams transported 18 horses in his trailer on January 28, with all but two of the animals loaded as two per stall.
He found the horses dead in the trailer once he arrived at his planned destination of Yarra Glenn in Melbourne's northeast.
Williams pleaded guilty to 16 counts of failing to ensure a horse was individually stalled and one count of using a method of management reasonably likely to result in unreasonable and unjustifiable pain and suffering.
Greg Barns, who represented Williams in Wednesday's hearing, said his client was devastated over the deaths, as he loved the animals he had worked with since he was a child.
Williams was genuinely remorseful for his actions, Mr Barns said, which was reflected by his guilty plea and decision to give up playing polo.
Mr Barns said Williams continued to teach and mentor young polo players, with plans to work in the United States, but a conviction would squander those job opportunities.
Prosecutor Madeline Wilson submitted that both TT-Line and Williams should be convicted of their crimes.
Ms Topfer will hand down her sentence on February 1 next year.
TT-Line has already lodged an appeal in the Supreme Court over its guilty verdict, which will be heard in a one-day hearing on February 27.
