The water supply causing widespread sickness in the Hawke's Bay was already under investigation for a previous contamination, the Herald can reveal.
More than 1000 people have been struck down by the gastric illness, with schools shut and businesses emptied in what's being labelled the largest such incident in modern New Zealand.
Two people remain in critical condition in hospital due to the outbreak, caused by a campylobacter infection in Havelock North's drinking supply detected late last week. The death of an elderly rest home resident has been potentially linked to the contamination.
It is the third time in three years there has been bacterial contamination in the same supply, which comes from bores drilled into the Te Mata aquifer 20m below ground.
E coli - a bacteria found in faeces - was discovered in the groundwater supply in both 2013 and 2015. After last year's contamination, the water was chlorinated and the bore responsible - known as "bore 3" - was closed, and an investigation began.
The Herald can reveal that, nine months later, the investigation was still not complete, with council documents showing its Works and Services Committee was yet to be informed of the possible causes of the previous contamination.
The investigation was being undertaken by environmental consultants Tonkin & Taylor. Bore 3 would remain closed until the report was complete, the documents said.
Bore 3 is less than 200 metres from the bores - "bore 1" and "bore 2" - at the centre of the current incident. All are on Brookvale Rd, north-west of the Havelock North township.
In a press conference today, council officials said they could not yet tell the cause of the latest contamination, although it was likely caused by surface water and linked to a recent rainfall event.
The council stressed the current bore was "somewhat distant" from the other bores and they hadn't been affected previously. Testing had shown they were in a slightly different part of the aquifer, and it wasn't known if the water was linked, the council said.
It said it would take experts some time to work out the cause of the latest incident, but agricultural and cropping activities were potential sources.
Officials are yet to answer questions from the Herald on why the previous investigation was ongoing, and whether the discovery of the source of the first contamination may have prevented the current outbreak.
Hastings mayor Lawrence Yule said earlier if the contamination source could not be found and eliminated, the Hastings District Council would have to permanently treat the bore system, or move the source away from the bores to another part of the Heretaunga Plains.
That is likely to have to happen anyway.
Until last year's contamination, Te Mata aquifer was thought to be a "secure" supply - so protected from outside contaminants - and therefore able to be consumed untreated.
However its use was affecting the nearby Mangateretere Stream, and the Hawke's Bay Regional Council had previously requested the council desist from drawing water there.
To comply with that request, last year the district council completed test drilling near Brookvale Rd in an attempt to find the deeper Heretaunga aquifer thought to run under the Te Mata source.
Despite finding a number of previously unknown aquifer layers, tests failed to identify a layer that would be a suitable supply. The council was still exploring other options for when its resource consent for Te Mata ended, documents showed.
The latest outbreak is now also under investigation.
Prime Minister John Key this morning met with the director-general of health and other officials over the outbreak.
Mr Key said the Ministry of Health would be involved in an inquiry to be run by the council and likely to be led by an independent judge. The council would do its own investigation.
Mr Yule has publicly apologised and promised an inquiry would find out what happened.
Earlier, professor of public health at the University of Otago Michael Baker told the Herald the incident was the "largest ever common-source outbreak" in recent New Zealand history.
While 100 years ago waterborne diseases like cholera and typhoid were very common, in modern times outbreaks were rare, he said.
The most recent had been in Darfield in 2012, when 125 residents were struck down with gastroenteritis after drinking contaminated water.
People who require urgent welfare support should call the council on 871 5000 and ask for the welfare line. The team will connect callers with the appropriate support team to address immediate needs.
For medical problems requiring non-urgent attention, phone the Healthline (0800 611 116) or a GP. For urgent health assistance phone 111.