The polo ponies were discovered dead in their converted refrigeration transport trailer following an overnight Bass Strait crossing from Tasmania to Victoria in January 2018.

TT-Line was found guilty in October of 29 animal welfare law breaches, while Williams pleaded guilty in July to 17 counts of breaching the animal welfare act.

During sentencing in Burnie Magistrates Court on Tuesday, TT-Line was fined $75,000 and Williams $15,000.

Magistrate Leanne Topfer said although there was no intent on the part of TT-Line, the company did nothing to check whether the horses were correctly individually stalled when they "squarely" had an obligation to.

"(This) is not a case of deliberate cruelty ... but society's attitudes have changed in recent times," she said.

She said Williams, who has been working with horses for three decades, failed in his duty to the animals.

Ms Topfer previously ruled the transport unit, which was stationary for 10 hours on a warm evening and carrying 18 horses, was clearly inadequately ventilated.

There were too many horses in the unit and the 16 who died were exposed to the risk of acute heat stress and asphyxiation, and suffered respiratory failure.

They were incorrectly "double stalled" while the two who survived were not.

Ms Topfer said TT-Line relied solely on a declaration by Williams, who was the driver of the horse transport, that the trailer complied with regulations.

Ms Topfer opted to record convictions against both parties, despite noting Williams' remorse and the abuse he'd received on social media following the incident.

TT-Line has lodged an appeal against the ruling in the Supreme Court of Tasmania, while Williams has ongoing separate civil legal action against TT-Line in the Supreme Court of Victoria.

Williams was unable to check on the horses, which were returning from a polo tournament in Tasmania, during the Bass Strait sailing as per TT-Line rules.

He had used the same truck and trailer eight times in the two years prior without difficulties.

Ms Topfer said Williams was devastated by the deaths and had sought help for his mental health.

His lawyers argued a recorded conviction would make it difficult for him to return as a polo player or coach.

TT-Line lawyers also argued a conviction should not be recorded because it would impact their reputation.

Ms Topfer said TT-Line, which is owned by the Tasmanian government, must have spent a "significant amount" defending the charges, including on one application that was "devoid of merit".

She said the fine was not a case where an individual was out of pocket.

Australian Associated Press