Some people with HIV/Aids in Papua New Guinea are being buried alive by their relatives
Margaret Marabe said families were taking the extreme action because they could no longer look after sufferers or feared catching the disease themselves.
Ms Marabe said she saw the "live burials" with her own eyes during a five-month trip to PNG's remote Southern Highlands.
PNG is in the grip of an HIV/Aids epidemic - the worst in the region.
Officials estimate that 2% of the six million population are infected, but campaigners believe the figure is much higher.
HIV diagnoses have been rising by around 30% each year since 1997, according to a UN Aids report.
Margaret Marabe, a known local activist in PNG, carried out an awareness campaign in the Tari area of the Southern Highlands earlier this year.
"I saw three people with my own eyes. When they got very sick and people could not look after them, they buried them," she told reporters.
She described how one person called out "mama, mama" as the soil was being shovelled over their head.
Villagers told her that such action was common, she said.
HIV/Aids is mostly spread in the country through heterosexual intercourse, and polygamy, rape and sexual violence are widespread.
Those caught up in the epidemic are often thought to be the victims of witchcraft.
Women accused of being witches have been tortured and murdered by mobs holding them responsible for the epidemic, according to officials and researchers.
Church leaders have described Aids patients being thrown off bridges or left to starve in back gardens in the past, the BBC's Phil Mercer in Sydney reports.
Ms Marabe, who works for the Igat Hope organisation in the capital, Port Moresby, said people in remote parts of the country remained ignorant about HIV/Aids and urged the government to take action.
"There are no voluntary counselling training centres in Tari. There are also no training programmes on HIV," she was quoted by PNG's Post-Courier newspaper as saying.
PNG's Secretary for Health Dr Nicholas Mann admitted to the BBC in an interview last year that the multitude of cultures and languages in the country made it difficult to get the HIV/Aids message across.
Papua New Guinea police to investigate claims AIDS patients buried alive