The first test showing elevated lead levels in Waikouaiti was received in August last year, says Dunedin City Council.
Yesterday, the council issued a notice to Waikouaiti and Karitane residents to not use tap water for drinking, cooking or preparing food until further notice as a precaution after it said elevated levels of lead in water sampling was found.
The council's infrastructure services general manager, Simon Drew, said six samples out of 90 taken over the past six months have returned results with elevated levels of lead.
Four of these times were at the Waikouaiti Golf Club - near the end of the supply line - once at the Karitane Bowls Club, and once at the Waikouaiti raw water reservoir.
The first of these was received on 13 August 2020, however after consultation and subsquent testing, the council said it had determined this was a "one-off spike".
"This is not completely uncommon, as lead can occur in water due to corrosion of old pipe fittings and private plumbing fixtures," the council said of the August test result.
At that stage, there was no evidence of the issue being widespread or continuing, it said.
It then put in place increased sampling and a 'pipe flushing' plan to remove metal traces from pipes.
"In consultation with the drinking water assessor, this action was considered appropriate until more data could be gathered."
During the following 14 weeks, lead tested well below the acceptable limits, except for another high result in October at the same location.
The next high result of lead was taken from samples taken on December, again at the golf club and also at the Karitāne Bowls Club.
Yesterday it was revealed these results were emailed to the council on 18 December, 2020 but went on unnoticed for a while.
"Unfortunately, because this sampling had been for asset management purposes rather than for drinking water standards monitoring, the email was sent to an inbox not being monitored while the staff member was on leave," the council said.
It acknowledged this would be "of little comfort to Waikouaiti/Karitane residents" and changes to their operations had been implemented.
"We have also put a procedure in place to ensure that the lab actively flags to us results which exceed safe drinking water guidelines."
The advice the council said it received after the email was picked up, was that more sampling and investigation was required.
"Public Health South concluded that public notification was not yet required."
The council said the most recent sample to indicate high levels of lead was taken at the Waikouaiti raw water reservoir on 20 January, with results coming in on Friday.
It was this that prompted the Medical Officer of Health to warn residents they should stop using the water supply for drinking, cooking or preparing food with.
Professor Ian Shaw from Canterbury University told Midday Report the response should have been quicker because cumulative amounts of high levels of lead can harm people's health.
Prof Shaw said people should have been told immediately, even if there was only one result showing high lead.
If people were drinking the water for a couple of days it would not have much effect, he said.
"But we're talking here from the 18 December or whenever it was right the way into February. That's quite a long time to be exposed to a high lead level.
"Lead's a cumulative poison so it gets taken in to the body but not excreted very well. If you take in a little bit each day it adds up in our body."
In the shorter term it can result in anaemia, and in the longer term it can cause changes in the brain, in particular developmental changes in the nervous system, he said.
"At levels below 10 micrograms per litre in water, even if you drink that for a long period of time you wouldn't expect any effect, but at the 39 micrograms per litre you could well expect some of these effects."
The metal usually comes from lead pipes and is normally at a consistent level in water, but can vary if the pH of the water changes, he said.