Australia on Saturday returned to Japan four sets of indigenous Ainu remains, more than 80 years since they were sent to the Pacific nation for research purposes.

The Japanese government has been stepping up efforts to bring back the remains of the indigenous ethnic group from Hokkaido, Japan's northernmost main island, which were collected -- sometimes by excavating tombs -- for studies in anthropology and other fields.

The remains, all skulls, were handed over to representatives of the Japanese government and Ainu-related groups in a ceremony at Melbourne Museum.

Masaru Okawa (R), who leads the Ainu Association of Hokkaido, thanks a person involved in Australia's  return to Japan of four sets of indigenous Ainu remains in Melbourne, Australia, on May 6, 2023. (Kyodo)

Masaru Okawa, who leads the Ainu Association of Hokkaido, told reporters later that he hopes to hold a memorial ceremony for the remains, saying they must have "felt very lonely" after being taken out of Japan for research purposes.

Inquiries by both countries found that researchers sent the remains to Australia between 1911 and 1936, including Yoshikiyo Koganei, an anatomist and anthropologist known for his Ainu studies and for being a professor emeritus at Tokyo Imperial University, which is now the University of Tokyo.

Three sets of remains were held by Museums Victoria in Melbourne, and one set was held by the National Museum of Australia in Canberra. The remains were kept in storage and never displayed, according to an official of the National Museum of Australia.

Addressing the Ainu association representatives present at the ceremony, Tim Goodwin from Museums Victoria expressed the museum's sincere regrets for the "indignities suffered" by the Ainu ancestors.

"We apologize for the distress their removal has caused your communities and sincerely hope that their return will help in repairing the damage caused," he said.

The remains include one skull found in 1921 in what is now the town of Kyowa in Hokkaido. Another was excavated in 1936 by researchers in the southern part of Sakhalin, north of Japan.

The place of origin of the two other sets of remains is unclear.

After arriving in Hokkaido on Monday, three sets of remains will be moved to a memorial facility at a major cultural complex dedicated to the Ainu people called Upopoy.

The fourth set of remains from Sakhalin Island will be held in temporary storage by a university in Japan while arrangements are finalized for their return to an Ainu organization.

Ainu people, who have their own customs and language, have lived for centuries in Hokkaido and nearby areas, including Sakhalin.

Remains of Ainu people taken out of Japan for anthropological research since before World War II have also been found in Germany, Britain and the United States. A German academic society returned a set of remains to Japan in 2017.