The federal government had no power to grant a permit for the live export of tens of thousands of sheep to the Middle East, the Federal Court has been told.
Animals Australia alleges the Department of Agriculture unlawfully gave Emanuel Exports the permit to ship 58,000 sheep from Fremantle in Western Australia on June 6.
Dan Star QC told Justice Debra Mortimer on Thursday the case could have ramifications for other live exports set to leave Australia over the coming months.
"This was such a case where there was no power to grant the permit at all," he said.
Mr Star told the court a permit could only be granted if it was found the livestock were healthy enough to undertake the voyage, and if there were adequate arrangements for the animals' health and welfare.
"The sheep on this voyage ... will suffer or (are) likely to suffer severe heat stress," he said.
"Some or many may die."
The legal action comes two months after Animals Australia released footage showing livestock in horrific conditions on an Emanuel Exports voyage to the Middle East in August 2017.
It led to a review by livestock vet Michael McCarthy of live sheep exports during the Middle East summer, which made 23 recommendations but did not call for the practice to be banned.
In court on Thursday, the agriculture department's lawyer, Tom Howe QC, said the Al Messilah vessel which left Australia on June 6 was required to have a vet employed by the department onboard.
But Mr Star said evidence showed the sheep would suffer heat stress if exposed to wet-bulb temperatures of 28C or more for a day or longer, adding there was a high probability of this during the voyage.
Outside court, the organisation's director of strategy Lyn White said "whether or not the sheep shipments depart to the Middle East during the summer months should not be a matter of Turnbull government policy".
"It should be a matter of what is lawful," she told reporters.
A trial date is yet to be set, but Animals Australia wants one as soon as possible due to various other imminent live export voyages.