It follows a Boil Water Notice issued by council on Monday morning.

Council staff spent the day delivering bottled water to schools in the affected areas, while residents snatched up bottled water at local supermarkets.

Dubbo Regional Council Director of Technical Services Stewart McLeod said the contaminants were found during regular testing “late last week”.

“Normal protocol is to retest,” Mr McLeod said.

“The retesting was done and the results came back over the weekend but those tests had failed as well.

“So there was a conference called this morning with public health authorities [and we issued the alert].”

Mr McLeod admitted a “small number” of residents could have been exposed to contaminants over the weekend, between the initial detection and Monday’s Boiled Water Alert.

But he denied council had put residents at about 3300 affected properties at risk by waiting until Monday to issue the alert.

“You need to make sure you're not getting a false positive because a Boiled Water Alert is not a trivial thing,” he said.

“It's not something to be done lightly.

“But once the security of our potable water is compromised we don't want people consuming the water until we can absolutely guarantee its safety.”

Water is safe for use in washing and on gardens. Water used for consumption should be brought to a rolling boil, cooled, stored in a clean container and refrigerated.

*NOTE: Commercial electric kettles with automated shutoff do not achieve a rolling boil for the required time frame and should not be used for this procedure.  We would advise boiling water in a saucepan rather than a kettle. 

Boiled or bottled water should be used for drinking, cooking, washing uncooked foods, making ice, personal hygiene, pet’s drinking water, washing hands, cleaning teeth, gargling, washing young children’s faces and washing children’s toys and utensils.

“The normal other uses can continue but you don't want to consume the water unless it's been boiled for about a minute to kill any [bacteria],” Mr McLeod said.

It is not yet known what caused the contamination.

“We're searching for the cause as you and I speak,” Mr McLeod said.

“We’re taking water samples...we do repeat tests.

“At the same time we’re flushing the mains in the areas that are affected, we were increasing the water in the treatment plant [and] we boosted the disinfectant which was a sensible protocol to take.”

He anticipated the Boiled Water Notice would remain in place until at least the end of the week.

“I would think most likely the end of this week would be the earliest that we'd be calling off the Boiled Water Alert,” he said.

“Worse case it continues until find whatever the problem might be.”

Dubbo Regional Council has offered an apology to affected residents.

Anyone with concerns they have fallen ill as a result of drinking contaminated water should consult their GP.

”You wouldn’t normally expect to get sick from the exposure that we think people might have had but that’s for individuals to make that assessment themselves,” Mr McLeod said.

Council will advise when the alert has been lifted through the media, website and social media.

EARLIER: Dubbo Regional Council advises that there is currently a Boil Water Notice in place for North Dubbo, bounded by an area west of the railway line to the Macquarie River, and Central and South Dubbo bounded by an area west of Fitzroy Street, Hampden and Palmer streets, to the Macquarie River.

This means that tap water is currently not safe to drink and may cause disease.

Water should be boiled for at least one minute before consumption by humans or pets.

Do not swallow water when showering and do not use to prepare food that will not be cooked.

The water may still be used for watering the garden.

This advice should be followed until the Notice is lifted. Council apologises for any inconvenience.