Officials in Si Maha Phot district were rushing to construct an emergency dyke around the industrial estate in tambon Tha Toom to prevent floodwater inundating all of its 500 factories.
Authorities said the Prachin Buri River burst its banks and immediately flooded areas around the industrial park.
Water also flooded parts of the park including a factory making car airbags.
Authorities said the water level in the factory was about 40cm deep. As the water flowed into the factory workers desperately tried to move manufacturing equipment to higher ground while pumps were used to drain water from the plant.
Authorities and park officials were Wednesday building a dyke around the industrial estate using large sandbags to prevent further flooding.
They said they feared the park could be completely inundated if heavy rain continues to fall over the next few days.
Prachin Buri has been hardest hit by downpours across the country since late last week.
Muang, Kabin Buri, Na Di, Prachantakham and Si Maha Phot districts have been flooded. More than 8,000 households have also been affected.
Jitra Promchutima, Prachin Buri governor, says the provincial office has declared the districts flood disaster areas.
She said she has instructed authorities to help provide food and drinking water for flood victims.
Ms Jitra said the water level in the Prachin Buri River in Si Maha Phot district on Wednesday rose to 9.80m, which was 80cm higher than the critical level.
She said provincial officials were speeding up efforts to install pumps to drain off a large volume of water in Si Maha Phot district and upper Kabin Buri district as quickly as possible.
Downpours have also severely hit several provinces in the Central Plains.
People living in communities along the banks of Chao Phraya River in Ang Thong, Sing Buri, Ayutthaya, Nonthaburi, Pathum Thani and Bangkok were experiencing flooding.
Deputy Interior Minister Wisarn Techathirawat said Prime Minister Yingluck Shinawatra will travel to Pathum Thani on Saturday to inspect the flood situation there.