The companion of an Indian medical student who died after being gang-raped on a bus in New Delhi was to begin giving evidence on Tuesday in the trial of five adults accused of her murder, his father said.

The 28-year-old, who cannot be named for legal reasons, arrived at the sessions court in New Delhi in a wheelchair, according to an AFP correspondent, and was expected to be the first prosecution witness to testify.

Although he did not speak to reporters, his father confirmed that his son would give evidence in the trial of the five men who have all denied murder, rape and robbery charges.

A sixth defendant is being tried separately as a juvenile.

"My son will go to any lengths to ensure that the guilty are punished," the father told AFP as the two of them entered the courtroom in the Saket district.

"He will cooperate and is prepared to answer any questions posed by the defence," he added.

The 23-year-old medical student died in a Singapore hospital on December 29 from massive internal injuries she sustained during the bus assault a fortnight earlier which caused outrage across India.

She and her male companion had spent the evening at the cinema and were lured onto the off-duty bus after failing to flag down an autorickshaw to take them home.

As well as taking turns to rape the medical student and violating her with a rusty iron bar, the group attacked her companion so badly that he is still unable to walk properly.

He is the main witness in a case that is being held in a special fast-track court.

The judge has banned all reporting of proceedings inside the courtroom and ordered lawyers not to speak to journalists.

[Image via Agence France-Presse]