Austrian technology company Andritz said Thai construction company CH. Karnchang had placed an order worth $324 million-$388 million for electromechanical equipment for the Xayaburi, following an international tendering process.
Andritz said start-up is scheduled for the end of 2019.
In collaboration with European experts, the Lao government has generally approved the project on the basis of accompanying measures addressing ecological and social aspects," Andritz said in a statement.
A ruling last December by the Mekong River Commission -- comprised of Laos, Thailand, Vietnam and Cambodia -- called for the project to be delayed until a comprehensive environmental study can properly identify potential risks.
The longest river in Southeast Asia, the Mekong stretches 3,000 miles to the South China Sea and is home to more than 700 species of freshwater fish, including the endangered Mekong catfish. The Lower Mekong supports nearly 60 million people who depend on it for their livelihood, says the World Wildlife Fund.
About 95 percent of the dam's 1,260-megawatt capacity is intended for export to Thailand, which is financing the project. Thailand would operate the dam, turning it over to Laos after 30 years.
Ame Trandem, Southeast Asia program director for California water rights group International Rivers, says Laos has no right to build the 1,285-megawatt dam because Cambodia and Vietnam haven't agreed to the project.
"Right now, it is impossible to say it is going to benefit the Lao people ... for past projects, there is no evidence that the revenue has benefited the community. Andritz should be reconsidering its involvement in this," Trandem was quoted as saying by The Phnom Penh Post.
Furthermore, conservationists say that Laos wants to build more than 70 dams along the Mekong's tributaries, much higher than the 11 dams that have long been known about, says a report in The Diplomat magazine.
International Rivers says that of those 70 dams, eight are under construction on the Xe Kaman and Xe Kong rivers and an additional 15 are intended for the Sekong River basin and a further seven on the Nam Ou River.
Lao Deputy Minister of Energy and Mines Viraphonh Viravong last week affirmed the country's commitment to hydropower.
"There is no question of (Laos) not developing its hydropower potential," he said in a Post report.