Police in Papua New Guinea have arrested a man for using a padlock to close his sister-in-law's genitals.

The woman called police in Lae, PNG's second-largest city, to say she was being assaulted.

When officers went to her house, they discovered the man who had moved into her house had closed her genitals with a padlock in December.

"When he moved in he started to rape the victim," Lae Police Superintendent Anthony Wagambie Jr said.

"Then he went on further to have her private parts padlocked and he had the key, he held on to the key himself."

Chief Superintendent Wagambie said the brother-in-law had been trusted to look after the woman and her family when her husband went to Port Moresby.

He said the man is likely to face multiple charges of rape and unlawful wounding.

'She had remained in fear'

The woman had been in great pain, but told police she was too scared to report what had been done to her.

"She didn't report out of fear," Chief Superintendent Wagambie said.

"The person had been threatening to kill her and he had brandished knives and pointed home-made guns at her and threatened to kill her and the children.

"So she had remained in fear."

Chief Superintendent Wagambie said the woman was now receiving counselling and treatment.

"This is very sad because she's a health worker who deals with these sort of cases of sexual and family violence and she advises people, but she also became a victim," he said.