Thai authorities said they were investigating why a single office tower block under construction in Bangkok collapsed during Friday's earthquake that otherwise caused limited damage in the capital.

Bangkok governor Chadchart Sittipunt visited the site on Monday as heavy equipment pulled away rubble from the 30-storey State Audit Office building in hopes of finding the 78 people still missing.

"Even one life saved is worth all the effort, so I think we have to move on, carry on,” he said.

But in the longer run it's important to ensure building safety in the city, where millions of people live and work in thousands of high-rise buildings, he said.

The magnitude 7.7 quake, which was centred more than 1,200 km away, killed more than 2,000 people in Myanmar and at least 18 in Thailand, most of them at the Bangkok construction site near the popular Chatuchak Market.

“I think we need to find the root cause so at least we can learn some lessons and improve building regulations," Chadchart said.

“In the end, we will have some results that will improve safety in Bangkok.”

Shares in the property developer handling the project, Italian Thai Development, fell 27 percent in trading on Monday as questions were raised about the building's design, enforcement of construction safety codes, and the Chinese contractor building the State Audit Office.

Interior Minister Anutin Charnvirakul, whose family owns one of Thailand’s largest construction conglomerates, said he has ordered an investigation committee to be set up and to report the results back to him within seven days.

He pointed to three possible factors behind the collapse: the designer, the inspectors or the builders. (AP)