He was one of the six scientists headed to attend the six-day 20th International AIDS Conference in Melbourne that started Sunday.
Lange's death has shocked the international public health and medical research communities, the scientific journal Nature reported.
"His contribution to HIV research and treatment and his determination to ensure access of those treatments for people in Africa and Asia, cannot be underestimated," said David Cooper, an AIDS researcher at University of New South Wales in Sydney.
Lange was a clinical virologist at University of Amsterdam.
His research explored HIV drug resistance, the role of anti-retroviral drugs in preventing transmission of virus from mother to child and other issues related to managing HIV AIDS.
His work helped to establish the safety and effectiveness of treating patients with multiple antiviral drugs, which is now standard.
In 2000, Lange helped launch the non-profit PharmAccess Foundation to bring anti-retroviral drugs to sub-Saharan Africa, where the HIV epidemic was in full swing and most patients remained untreated.
The foundation has since broadened its reach, creating a fund that subsidises health insurance for about 1,00,000 people in Nigeria, Tanzania and Kenya.
"This is a massive loss. We are devastated. Joep's dedication to the treatment of HIV AIDS and global health in general has been ground breaking," said Onno Schellekens, managing director of PharmAccess.
"He was very selfless. He believed we have made great scientific progress and people have to benefit from it," Robin Weiss, a virologist at University College London, emphasised.