WORKERS at the Toowoomba Second Range Crossing site were drinking water containing E. coli and enterococci bacteria for at least six days.
Elevated levels of the bacteria are indicative of faecal contamination.
Construction consortium Nexus was first alerted to the potential for contamination on February 15, when the company was issued with an improvement notice from Workplace Health and Safety Queensland, which a WHSQ spokesman said was "in relation to the provision of drinking water, including the quality, cross contamination risks, signage and availability".
The Construction, Forestry, Mining and Energy Union said a number of workers had recently taken sick.
Workers contacted The Chronicle after Nexus told crews their drinking water was contaminated on Monday, but would not explain further.
Nexus provided the results of water tests conducted on Tuesday, February 20, to WHSQ late on Wednesday.
The results showed elevated levels of the bacteria E. coli and enterococci in drinking water at four of 13 worksites.
The WHSQ spokesman said Nexus had advised water at the four sites was "immediately flushed, disinfected and chlorinated".
WHSQ inspectors, including an occupational hygienist were on site yesterday, and WHSQ has taken samples to conduct its own water testing.
A Nexus spokeswoman said no related sickness had been reported to the onsite paramedic in the past two weeks.