The decision to fly the victim of a gang-rape in India for treatment in Singapore has been criticised by doctors.

They say it made little medical sense as the woman was so severely injured that her death was all but inevitable.

The government has struggled to defend its decision to send the 23-year-old physiotherapy student overseas. She died 48 hours later.

It is also on the back foot after street protests and stinging criticism of authorities over the 16 December rape in New Delhi.

Doctors said that the woman was just clinging to life when she was flown 4,000km from New Delhi to Singapore.

They said a deadly infection seeping into her blood from damage done to her intestines during the assault was complicated by a cardiac arrest and damage to the brain.

"It was ethically and morally wrong to have taken her out, given that she was sinking and her chances of survival were next to zero at that stage," said a doctor at All India Institute of Medical Sciences.

The institute was advising the team treating the woman at a sister hospital across the street.

"Such a thing raises false hopes in the minds of the family, the community. No doctor in his right mind would do this, unless you want to get the patient off your back," said the doctor, who declined to be identified.

He said colleagues at the government-run hospital who had spoken out had been warned of consequences in the politically explosive case.

The woman, who was assaulted by five men and a teenager on a moving bus after a male companion was beaten unconscious, cannot be named under an Indian law that prohibits identifying victims of rape.

Another doctor who was consulted during the woman's care at New Delhi's Safdarjang hospital, where she was taken following the assault, said she had been getting the best possible treatment in India and the question of why she was moved should be answered by the government.

Many security officials have said they feared the protests would escalate if the woman had died in New Delhi, but the government has said the only consideration was her wellbeing.

"The idea was to give her the best possible treatment," said Harish Rawat, a government minister who attended a cabinet meeting on the woman's condition and the efforts to save her.

"We felt if there was a chance to save her, it should be tried. Take her to a transplant facility abroad," he said.

At the time of the transfer, authorities at Safdarjang said her condition was critical, which was why they decided to move her to Singapore's Mount Elizabeth Hospital, which specialised in multi-organ transplant.

But doctors said a transplant for her damaged intestine, if at all possible, was months away.