Staff knocked on the doors of about 370 customers on Saturday to warn them they were at risk of diarrhoea or severe stomach cramps.

They are also arranging for supplies of free drinking water for people in Harcourt.

The scare has been caused by potentially harmful levels of the E. coli bacteria in part of the region’s water supply.

The advice was given to Coliban customers in Harcourt, Barkers Creek and north Castlemaine.

Managing director Gavin Hanlon said the bacteria was detected in routine sampling in one of the sample sites on Friday, so Coliban flushed the system and did more extensive sampling.

The advice to boil water was issued on Saturday after samples indicated the E. coli was still in the system.

E. coli is an indicator species that is present when the water is contaminated.

Mr Hanlon said the contaminate could range from animal faeces to raw water that had found its way into the system.

Coliban believes the outbreak was caused by an animal breaking into the reservoir and puncturing a hole in one of the bladders, letting contaminated water spill into the system.

“We suspect that’s the case, but we’ll continue to sample,” Mr Hanlon said.

Coliban was also in the process of organising free bottled water to be available at the Harcourt general store.

“We’re doing everything we can to fix the problem and have safe drinking water back to that part of the world.”

Mr Hanson said customers might also experience low water pressure as the distribution system was being disinfected and flushed as a result of the contamination.

Residents may notice discolouration due to the system being flushed and are advised to run their taps until the water runs clear. For more information, customers can telephone Coliban Water on 1300 363 200 or go to www.coliban.com.au