At least 91 people were missing after a giant mound of mud and construction waste spewed out of an overfull dump site in a southern China boomtown and buried 33 buildings in the country's latest industrial disaster.

The site should have been closed down in February, but according to local workers, mud and waste had continued to be dumped there, a news portal run by the city government in Shenzhen said.

Premier Li Keqiang ordered an official investigation into Sunday's landslide in Shenzhen, just across the border from Hong Kong. The mudslide smashed into multi-storey buildings at the Hengtaiyu industrial park in the city's northwestern Guangming New District, toppling them within seconds in collisions that sent rivers of earth skyward.