Death threats have been made against the Australian radio hosts involved in the royal prank call tragedy, police said Friday, with station management reportedly moving some staff to safehouses.

The revelations came as a London inquest showed the nurse who fielded the hoax call, Jacintha Saldanha, 46, hanged herself.

The mother-of-two was found dead last Friday, three days after transferring the call to a colleague who divulged details about Prince William's pregnant wife Catherine who was recovery from severe morning sickness.

Saldanha was discovered in nurses' quarters near the private King Edward VII's Hospital in central London where Kate was being treated. She also had wrist injuries and left three notes.

Australian police have launched an investigation into the death threats after a letter targeting presenter Michael Christian was seized, warning him there were "bullets out there with your name on it".

The letter was obtained by Sydney's Daily Telegraph, which said further threats were made involving a shotgun which it said were inappropriate to print.

The Australian Broadcasting Corporation said staff at 2Day FM's parent company Southern Cross Austereo have been receiving threats all week.

"Police are conducting an investigation into threats made against two Sydney radio presenters," a New South Wales police spokesman told AFP.

"Detectives seized a letter which contained a number of threats. Detectives are conducting an investigation into the matter and are attempting to identify the source of the letter."

Southern Cross Austereo could not reached for comment but the Telegraph said about a dozen staff at the broadcaster have been moved to hotels for their safety and up to 10 executives have been assigned bodyguards.

Christian and fellow host Mel Greig, who have been undergoing counselling, remain off air and have not been seen in public since making tearful apologies on commercial television on Monday.

It follows the pair posing as Queen Elizabeth II and William's father Prince Charles in a hoax call to the hospital, which sparked an outpouring of global anger against them.

Senior British police officer James Harman told the inquest there were "no suspicious circumstances" surrounding Saldanha's death, indicating that no one else was involved.

"Jacintha Saldanha was found by a colleague and a member of security staff. Sadly she was found hanging. There were also injuries to her wrist," he told Westminster Coroner's Court.

Two notes were found in her room and another was among her possessions, Harman added, without revealing their contents at this preliminary stage.

Police are also looking at telephone calls and emails to see if they shed more light on her death, he said.

Scotland Yard will "in the very near future" be in contact with Australian police to ask them to interview people there, he added.

Indian-born Saldanha's husband Benedict Barboza and two teenage children did not attend the short hearing.

The radio station has pledged at least Aus$500,000 (US$523,600) to help the grieving family, although British lawmaker Keith Vaz, who has been acting as their spokesman, said the broadcaster had not done enough.

The royal couple, who are expecting their first child, have said they at no stage complained to the hospital about the hoax call incident. The hospital has also said that it gave Saldanha its full support.

Australia's media watchdog on Thursday opened an investigation into the prank call to determine whether it breached standards of decency, invaded privacy or contravened the Commercial Radio Codes of Practice.