Flight MH17 from Amsterdam to Kuala Lumpur, which US officials believe was hit by a surface-to-air missile over Ukraine, killing all 298 people on board, was due to connect with another flight to Melbourne.
The Sydney Morning Herald said as many as 100 of those who died were delegates en route to Melbourne for the 20th International AIDS Conference, which is due to begin on Sunday.
The Australian broadsheet reported 108 attendees and family members were killed, including prominent former International AIDS Society president Joep Lange.
Also on board was Glenn Thomas, a spokesman for the World Health Organisation, who was similarly headed for the conference, the UN agency said Friday.
Denis Napthine, the premier of Victoria state which is hosting the conference, said a "substantial" number of those attending were on the plane.
"We know it has been confirmed a number of senior people who were coming out here -- researchers, medical scientists, doctors, people who have been at the forefront of dealing with AIDS across the world -- were on that flight," he said.
The exact number was not yet known, he said, "but there is no doubt that it is a substantial number".
The International AIDS Society confirmed that "a number of our colleagues and friends" were killed, but did not say how many.
Asked by reporters whether 108 people attending the conference were on the flight, International AIDS Society president Francoise Barre-Sinoussi said she was not sure.
- 'They would want conference to go on' -
"We don't have the confirmation (of numbers)," said the Nobel laureate.
In a tweet, UNAIDS executive director Michel Sidibe said "many" delegates were on board.
Held every two years, the International AIDS Conference is a forum for campaigners to highlight developments in fighting the disease and discuss financing problems.
It was this year also expected to channel anger about laws in Africa that stigmatise homosexuality and in the former Soviet Union that punish intravenous drug users -- a crackdown now extended to Russian-annexed Crimea.
Barre-Sinoussi said it would go ahead despite the tragic news.
"The decision to go on, we were thinking about them because we know it's really what they would have liked us to do," she said.
Some 12,000 participants are due to take part, joined by former US president Bill Clinton and rock singer and poverty activist Bob Geldof.
Barre-Sinoussi paid tribute to Dutchman and leading AIDS researcher Lange, who has been involved in HIV research and treatment since 1983, saying he had dedicated his life to "the benefit of mankind".
Officials from the PharmAccess Foundation, which Lange launched in 2000 to facilitate access to treatment for HIV and AIDS patients across Africa, said his death was "a massive loss".
Lange's "dedication to the treatment of HIV/AIDS and global health in general has been groundbreaking," PharmAccess managing director Onno Schellekens said in a statement.
WHO officials also marked the loss of Thomas, the agency spokesman. "It is with deep sadness that WHO lost one of our colleagues in the Malaysia crash," communications official Gregory Hartl told reporters in Geneva of the Briton.
"His twin sister said he died doing what he loves," Hartl said, adding that no other UN staff were aboard the doomed airliner.
American academic and AIDS activist Gregg Gonsalves tweeted that there were "lots of AIDS researchers, activists, officials on downed Malaysia Airlines flight to Melbourne for Intl AIDS Conference".
Of the 298 onboard the plane, 154 were Dutch nationals. There also were 43 Malaysians and 28 Australians, as well as passengers from Indonesia, Germany, Belgium, the Philippines, Britain and Canada.
London-based AIDS campaigner Mark Gettleson said some of those on the flight were from the STOPAIDSNOW group, which works to offer care and treatment to those affected by HIV and AIDS, and support prevention initiatives.
The flight took off from Schiphol airport in Amsterdam and was supposed to land in Kuala Lumpur early Friday. A scheduled connection to Melbourne was due to arrive Friday evening.
Some 35 million people live with HIV, although global AIDS-related deaths and new infections have fallen by more than a third in a decade, raising hopes of beating the killer disease by 2030, the United Nations said Wednesday.