Watercare is ramping up efforts to determine how a monitoring tap became contaminated with E. coli, triggering a boil-water notice last week that affected thousands of Auckland households and temporarily shuttered businesses.

After routine water testing on April 6 detected E. coli in one of the samples taken, Watercare issued a “precautionary” boil-water notice for about 7800 properties across parts of Hillsborough, Mt Roskill, Royal Oak and Three Kings.

It was lifted about 36 hours later, with Watercare’s investigations concluding the detection was tied to “a specific water quality monitoring tap” and that the water supply was unaffected.

Questions now linger over how E. coli, which can cause severe illness such as diarrhoea and vomiting, was able to take up residence in the tap but not the wider network.

Watercare’s chief operations officer Mark Bourne told the Herald Watercare replaced the contaminated tap while the boil-water notice was in effect, and the council-owned entity remained confident there was no risk of wider contamination in the water supply.

Ongoing samples from the new tap – all of which have fallen within normal ranges with no E. coli detected – have reinforced Watercare’s conclusion it was a localised contamination.

A custom-built sample tap used by Watercare for routine water sampling and monitoring. The tainted tap is being microbiologically tested and inspected for flaws, chief operations officer Mark Bourne said. Photo / Watercare

“We are having the tap that was removed microbiologically tested and inspected for any internal damage or flaws,” Bourne said.

“It will first be tested by our laboratory and then by an external materials specialist.

A definitive timeframe for the results could not be given because of the “comprehensive process” involved, he said.

Bourne said the monitoring taps – custom-built metal spigots found citywide to monitor water quality in different areas – are securely installed in cabinets and used exclusively to collect samples.

When sampling technicians open the cabinets, the taps become exposed to the outside environment, and contamination could potentially be introduced from conditions like wind or rain.

Watercare lifted the boil-water notice last week but will continue to run tests and inspections on the tainted tap. Photo / Michael Craig

“This is normally mitigated through strict sampling protocols such as hand hygiene, flaming of the tap outlet using a gas cylinder with a flame nozzle to sterilise it, and flushing the tap before the sample is taken,” Bourne said.

“Although very rare, sometimes these mitigations may not be enough, and whatever has contaminated the sample tap may not have been removed before the sample was collected.”

The boil-water notice was lifted on the morning of April 8, with Watercare assuring residents the supply was “now safe to use as normal”.

The following day, New Plymouth District Council issued its own notice for residents in the city’s Bell Block and The Links areas after a water sample tested positive for E. coli.

The New Plymouth notice was gradually lifted over the weekend as water samples came back clear, but the cause behind the positive result remains under investigation.