The accident took place at 9:40 am (local time) in a remote village in Suichang County in Lishui City.
The vehicle was carrying seven passengers including three geologists who were on their way to conduct an official survey.
The driver and one of the villagers reported the incident after managing to reach the shore, state-run Xinhua news agency reported.
Over 60 firefighters, police officers and rescuers are trying hard to find the missing people.
The minibus has been located in the reservoir. It's eight to nine meters deep, but the missing passengers are yet to be found, the locals said.
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Road accidents are a serious problem in China.
According to World Health Organisation (WHO), more than 250,000 people are killed in road accidents in China.
The country's frequently overcrowded long-distance buses are prone to accidents, with individual incidents regularly causing dozens of deaths.
Road injuries are the third leading cause of premature death in China, ranking above any individual form of cancer, according to the Global Burden of Disease Study, a global research programme.