Otago Regional Council chief executive Sarah Gardner said the worst result in the Lower Waitaki plains area was from March.
"The levels have spiked at quite a high number - I understand at around 150 [colony forming units - or viable cells - per millilitre] above what the drinking water standard is.
"At other times, they have been relatively close to the standard - but not as close as they should be."
The safe level for drinking water is "less than one" colony forming unit per millilitre.
"Bore owners had been sent emails warning them not to drink the untreated water and would receive follow up letters in the post," Mrs Gardner said.
The council was still investigating the source of the contamination.
The Southern District Health Board Medical Officer of Health, Keith Reid, said residents must not drink untreated water or use it for cooking. Read more