Amnesty said Papua New Guinea police were investigating the incident, but police in Port Moresby were unable to confirm this to AFP.
It comes just weeks after a woman accused of sorcery was burned to death by a mob.
A report in The National newspaper said that six women and a man accused of sorcery were tortured as Easter “sacrifices” in a village in the Southern Highlands on March 28.
The man, Komape Lap, 54, told the newspaper he fought with his attackers and escaped but did not know what had become of the six women, two of whom were his wives.
He said the women had their hands tied, were stripped naked and had hot irons placed “into their genitals” during the ordeal.
Amnesty called on authorities in PNG to prevent and punish such violence.
“The priority must be to find out the fate of the six women,” Amnesty International’s Pacific researcher Kate Schuetze said.
“The perpetrators must also be brought to justice for the abduction and crimes of sexual and other violence, if confirmed.”