Police in Papua New Guinea regularly rape and torture children despite efforts to reform the juvenile justice system, Human Rights Watch said in a report released on Monday.
"Police rapes and torture are crimes, not methods of crime control," said Zama Coursen-Neff, senior researcher for Human Rights Watch's children's rights division.
"These brutal tactics have destroyed public confidence in the police."
The New York-based organisation said police also routinely locked children up with adults, putting them at risk of rape and violence, and rarely provided medical care for those who were injured.
The 50-page report, "Still Making Their Own Rules: Ongoing impunity for police beatings, rape and torture in Papua New Guinea", is a follow-up to a 2005 report on police violence against children.
The report said abusive officials rarely faced punishment, leaving ordinary people in the impoverished half-island South Pacific state as afraid of the police as they were of criminals.
Police abuse may also contribute to Papua New Guinea's rapidly escalating HIV epidemic, the report said.
"With an estimated 140,000 people living with HIV/AIDS, Papua New Guinea has the highest infection rate in the South Pacific.
"Police abuse -- particularly the targeting of sex workers, and boys and men perceived to be homosexual, as well as harassment of people carrying condoms -- may worsen the epidemic by undermining HIV prevention efforts."
The report said there were small signs of progress in the juvenile justice system, including the establishment of a two-person unit to monitor police treatment of children, but they had failed to produce noticeable change.
Human Rights Watch pointed out that Australia was Papua New Guinea's largest foreign aid donor, but that direct aid to the police remained under negotiation after the withdrawal of Australian police in 2005.
"Papua New Guinea and Australia should make sure that protection for the human rights of children and marginalised groups is at the forefront of future assistance to the police," said Coursen-Neff.