"He was a kind man and a true humanitarian,'' Seema Yasmin, a United States medical professor, said in a series of tweets.
"How do we measure how much a person has done for humanity? People like Joep change the course of epidemics ... What a HUGE loss to the world.'' The International Aids Society said that Dr Lange's death meant "the HIV/Aids movement has truly lost a giant.
"At this incredibly sad and sensitive time the IAS stands with our international family and sends condolences to the loved ones of those who have been lost to this tragedy," it said in a statement.
The conference in Melbourne from July 20 to 25 featured the world's leading researchers, as well as advocates and guests such as Bill Clinton and Sir Bob Geldof.
Delegates at a pre-conference in Sydney were reportedly informed that 108 delegates were aboard the flight.
"It's going to be a very sombre mood at the conference in Melbourne, especially for those of us who have been coming to these conferences for many years," Clive Aspin, a health researcher, told Fairfax Media.
David Cooper, a leading Australia HIV researcher, said he had been a friend and colleague of Dr Lange for 30 years.
"[He had] an absolute commitment to HIV treatment and care in Asia and Africa," Professor Cooper told The Conversation.
"Joep was absolutely committed to the development of affordable HIV treatments, particularly combination therapies, for use in resource-poor countries. The joy in collaborating with Joep was that he would always bring a fresh view, a unique take on things, and he never accepted that something was impossible to achieve."