Sheep on a live export shipment during the high-risk northern summer months were made to stand in pens next to oil fuel heaters and some died due to smothering when crowding around air vents, a report released by the federal agriculture department says.

Six hundred sheep died on the voyage of the Kuwait-flagged ship Al Shuwaikh from Australia to the Middle East in May and June last year.

A summary of the report written by the independent observer was published on the department’s website on Thursday. The RSPCA has questioned why the report was not released for 11 months, despite containing information about heat stress that may have influenced the design of new live export rules.

It said the report was “a major indictment” on the decision of the government to continue the live export trade between May and June this year, despite calls from animal welfare groups, the West Australian government, and Labor and the Greens to stop it. Labor has promised to ban the trade if elected.

“This report was in the department’s possession in June 2018 – to withhold its release until after the regulations for this northern summer period were made is very concerning indeed,” RSPCA senior policy officer Dr Jed Goodfellow said. “It shows, yet again, that the Department of Agriculture cannot be trusted to effectively regulate live exports and that a truly independent regulator is urgently required.”

The Al Shuwaikh left Fremantle on 15 May, two days before agriculture minister David Littleproud released a review into the management of heat stress on live export ships.

About 0.88% of the 69,117 sheep on board died — just above the average for a live export shipment — but all 263 cattle survived.

The observer’s own report, released in heavily redacted form under freedom of information laws to the RSPCA in January, said that poor communication between crew members “resulted in some pens being without water overnight on numerous occasions”.

But that observation was not included in the department’s public summary report, which said the watering system was “efficient and well maintained”.

The version released under FOI also mentioned the “training, or lack of it” among the crew, which the final report omitted, saying instead that all crew had “up to 10 years experience” and were “kind when handling livestock”.

Both versions of the report said that the closed decks of the ship were hotter and more humid than the open decks and attributed that to a fuel heater, which was turned off when the ship reached the equator.

Both also said that sheep on deck nine had elevated respiratory rates because the steel roof above them was painted a dark colour and absorbed the heat. Elevated respiratory rates and panting are signs of heat stress.

The department’s report said sheep were “open mouth breathing and attempting to gain position around the ventilation vents on all open and closed decks” on eight days of the voyage, adding that “in these instances, death by smothering was an observed outcome”.

But it said the voyage complied with the Australian standards for the export of livestock.

The department defended the delay in releasing the report and the discrepancies between the report as released under FOI and the public summary.

“This summary report accurately reflects the key observations made by the independent observer,” a department spokesman said. “It was fact-checked and agreed to by the independent observer prior to publication.”

The department said it published reports “as soon as practicable” and that timeframe could vary “depending on a number of factors”.

Asked why the detail about animals being left without water overnight was not included in the summary, the department said: “The [independent observer] reported no adverse animal welfare outcomes as a result of this incident.”