A panel of judges from International People’s Tribunal on 1965 Crimes Against Humanity in Indonesia met in November in The Hague and has released its findings.
It says up to 500,000 people were killed during a Communist purge by the Indonesian government in 1965.
The panel said other crimes against humanity - including inhumane imprisonment, enslavement, torture and sexual violence - also took place.
It said it was part of a widespread systematic attack on the Partai Komunis Indonesia (Communist Party of Indonesia) and those deemed to be connected with it.
The report also found that other countries, specifically the United States, Britain and Australia, were complicit in facilitating the wrongful acts of mass killings and other crimes against humanity.
It also accuses Australia of running what it describes as a sophisticated propaganda operation.
“The UK and Australia conducted a sustained campaign repeating false propaganda from the Indonesian army, and that they continued with this policy even after it had become abundantly clear that killings and other crimes against humanity were taking place on a mass and indiscriminate basis," it said.
"On balance this justifies the charge of complicity in the above crimes against humanity," the panel added.
It's called on the Indonesian government to apologise to the victims, survivors and their families in relation to the 1965 events.
It's also called for the investigation and prosecution of all crimes against humanity and to ensure appropriate compensation and reparation to victims and survivors.
The report is not legally binding.
Indonesia did not participate in the hearings. Australia, Britain and the United States also didn't take part in the proceedings which took place in The Hague over four days between November 10 to November 13.
It says it heard from the prosecutors, as well as from more than 20 witnesses.
The judges also received several hundred pages of documents which were tendered as evidence.
But a spokesman for Australia's Department of Foreign Affairs and Trade told SBS that the International People’s Tribunal was "human rights NGO" not a "formal international court or tribunal".
"Australia rejects any suggestion that it was complicit in any way in those events of 50 years ago," the spokesman said.
"It is for Indonesians to address and settle these historical events amongst themselves. Australia and Indonesia now focus on further developing a strong bilateral relationship based on shared interests in security, trade, investment, tourism, education, development and cultural affairs."
