Sixteen polo ponies found dead after a Spirit of Tasmania summer sailing across Bass Strait "suffocated" during the voyage inside a trailer where they were incorrectly stalled without adequate ventilation, a court has been told.
Ferry operator TT-Line has pleaded not guilty to a host of animal welfare-related charges in relation to the ill-fated trip in January 2018.
Crown prosecutor Simon Nicholson told Burnie Magistrates Court autopsies on three of the horses found their injuries were consistent with sudden death due to respiratory failure.
Mr Nicholson said the trailer, a converted refrigeration transport, had six permanently open vents down the side, no mechanical ventilation and relied on passive airflow.
He said a veterinary expert would give evidence there was a "high probability" of the ponies suffering heat stress from their own body heat.
"The temperature was, at best, very uncomfortable," he said during opening submissions on Wednesday.
Mr Nicholson said the ponies appeared "happy" when they were loaded into the trailer at Barnbougle in Tasmania.
They were among 18 horses on one of two trailers heading back to Victoria after completing a polo tournament.
The ferry arrived in Melbourne about 5.40am on January 29. The dead ponies were discovered when the trailers were opened at Yarra Glen in regional Victoria around 7am.
Mr Nicholson said the horses were incorrectly "double stalled" in the trailer, and there only should have been 11 inside.
He said the deaths occurred between 2am and 4am.
The two surviving horses were treated for a high heart and respiratory rate and appeared to have suffered unreasonable pain, he said.
TT-Line has pleaded not guilty to using a method of management reasonably likely to result in unreasonable and unjustifiable pain and suffering.
It has also pleaded not guilty to 28 counts of failing to ensure a horse was individually stalled.
Mr Nicholson said the hearing, slated to run for three weeks, would hear evidence from a vet that the horses died from asphyxiation due to suffocation.
He said a contributing factor was inadequate vents that did not comply with TT-Line standards.
"TT-Line had care and charge of the horses ... it need not matter that the affected trailer completed several successful trips before," he said.
Mr Nicholson said it was an otherwise "uneventful" voyage and temperatures were in the 20s throughout the evening.
The hearing continues on Thursday.
