A few simply expired from old age, but in many cases the blame was ascribed to mysterious gastro-intestinal ailments or misadventure.

The latest casualty was a double tragedy - Kua, a four-year-old one-horned rhinoceros, was pregnant when she died last week.

The incident was something of an international embarrassment. Kua had been brought only last year from San Diego Zoo in the United States, and the birth of a one-horned rhino calf would have been a coup.

Dark rumours are swirling around her death. A local MP, Lee Rhiannon from the Greens party, claims that keepers mistook Kua's pregnancy for her being overweight and stopped feeding her. Out of desperation, Kua turned to chewing on her bedding, Ms Rhiannon says.

'The alarm bells have been rung loud and long about the botched management of rare and endangered species at Taronga,' she says.

The zoo has dismissed her allegations as ridiculous. Managers say Kua died as a result of a blockage to her gut caused by eating sand but admit they have no idea where the sand came from.

An animal welfare organisation, Humane Society International, has called for the result of the investigation to be made public to determine 'who or what is culpable' in the rhino's death.

Taronga has a sister facility, Western Plains Zoo at Dubbo, in the middle of New South Wales. It, too, has been struck by tragedy, with the death in the past 10 days of an elephant named Cheri.

Both deaths are now being investigated by the Royal Society for the Prevention of Cruelty to Animals.

The RSPCA points out that Taronga cares for more than 4,000 animals, and deaths from illness or old age are inevitable. But it concedes that it is worried by the recent run of deaths.

'Yes it's a concern. Fifteen animals - that's why we're investigating,' said deputy chief executive Steve Coleman.

Taronga chief executive Guy Cooper says that 'everything humanly possible' was done to try to save the animals, and that allegations of negligence are a slur on the hundreds of keepers and carers who work at the two zoos.

But there are broader concerns that large creatures such as rhinos and elephants have no place in a zoo like Taronga, which was built in the 19th century.

Despite a recent multimillion-dollar redesign, including a A$40 million (HK$260 million) rainforest habitat, it remains cramped for space, surrounded on all sides by upmarket suburbia.

Despite the best efforts of staff, some animals have lapsed into the stereotypical behaviour of caged or bored animals, with visitors often distressed by the sight of a sun bear pacing in its enclosure, and of an old elephant constantly swaying its head back and forth.

Taronga has only just emerged from a bitter two-year legal fight with animal welfare groups over the importation of wild elephants from Thailand. Organisations such as the RSPCA and the Humane Society argued that transporting the elephants from Thailand to Australia, with a period of quarantine on the Cocos Islands in the Indian Ocean, subjected the animals to great stress.

They argued that the benefit of bringing the pachyderms to Australia was limited, as no Asian elephant in captivity has yet successfully bred in this country.

Such was the anger over the elephants' arrival in Sydney that they had to be given a police escort after arriving on giant Russian transport planes.

Besides the RSPCA investigation, the New South Wales state government has demanded a report on Kua's death from Taronga's management. Due to be handed over at the end of this week, it will be scrutinised by all sides.