The woman, named by The National newspaper as Kepari Leniata, 20, was reportedly tortured with a branding iron and tied up, splashed with fuel and set alight on a pile of rubbish topped with car tyres.
According to the rival Post-Courier newspaper she was torched by villagers who claimed she killed a six-year-old boy through sorcery, with police outnumbered by onlookers and unable to intervene.
A fire truck that responded to the incident, which took place yesterday morning in Mount Hagen city in the Western Highlands, was also chased away.
According to the reports, which were accompanied by graphic front-page images of the woman's burning corpse, she admitted to killing the boy, who died after being hospitalised with stomach and chest pains on Tuesday.
Police said they were treating the torching as murder and preparing charges against those responsible.
There is a widespread belief in sorcery in the poverty-stricken Pacific nation where many people do not accept natural causes as an explanation for misfortune, illness, accidents or death.
In 1971, the country introduced a Sorcery Act to criminalise the practice. But PNG's law reform commission recently proposed to repeal it after a rise in attacks on people thought to practise black magic.
Local bishop David Piso said many innocent people had been killed.
"Sorcery and sorcery-related killings are growing and the government needs to come up with a law to stop such practice," Piso told The National.
Police arrested dozens of people last year linked to an alleged cannibal cult accused of killing at least seven people, eating their brains raw and making soup from their penises.
There have been several other cases of witchcraft and cannibalism in PNG in recent years, with a man reportedly found eating his screaming, newborn son during a sorcery initiation ceremony in 2011.
In 2009, a young woman was stripped naked, gagged and burnt alive at the stake, also in Mount Hagen, in what was said to be a sorcery-related crime.