Shui Meng Ng says she hopes Laotian leaders will not brush off concerns about her husband of more than 30 years with the response that police are still investigating.

"I hope this time they will show some good will and sincerity by agreeing to accept international assistance and conduct a serious and transparent investigation into what happened to Sombath," Ms Ng told the Foreign Correspondents' Club of Thailand.

"Maybe it is pie in the sky. But whatever the response, I can never give up hope and I will not be deterred from my search," she said.

US-educated Mr Sombath, an environmental activist who has won awards for his work improving the lives of Laotian farmers, including helping those left homeless and without compensation in land deals, was abducted from a police checkpoint in the capital Vientiane on the evening of December 15, 2012.

His disappearance shocked many Laotians even in a country where the ruling Lao People's Revolutionary Party has a history of crushing any threats to its monopoly on power.

The United Nations Working Group on Enforced or Involuntary Disappearances has evidence of eight similar cases.

Mr Turnbull will be among 18 leaders attending a series of Association of South-East Asian Nations summit meetings in Vientiane for three days this week.

Mr Obama is the first sitting US president to visit the mountainous and landlocked country of 6.8 million people, which in recent years has drifted into China's sphere of influence.

Phil Robertson from Human Rights Watch said Laos has gone to great lengths to track down and arrest critics, strictly control the media and muzzle civil society groups with intimidation and regulatory controls, making it one of the most repressive regimes in the region.

"The case of Sombath Somphone is front and centre in the international community's concerns about rights and governance in Vientiane and Prime Minister Turnbull and other world leaders should take this up with their hosts," he said.

"Australia is an important bilateral donor to Laos and it should use that leverage to press for real improvements in human rights there."

Ms Ng, who is from Singapore and met Mr Sombath while they were studying in the US, said for more than two years no Laotian officials have contacted her about her husband's disappearance.

"The lack of any new information has been psychologically and emotionally draining. Not only is it extremely difficult to bear, it is also increasingly challenging for me to sustain support from the diplomatic community, the UN, the media and even Sombath's friends and family inside and outside Laos," she said.

"I believe that is exactly what Sombath's abductors want … the passing of time to erase the memory of Sombath's disappearance."