A draft report on a 2013 contamination of Havelock North's water supply has said earthworks on a neighbouring mushroom farm may be to blame.
In October last year, E. coli was found in one of three bores providing water to Havelock North.
The bore is on the same road as the two bores at the centre of the current outbreak, but was not being used.
In the 2013 incident Havelock North water was chlorinated for several days and no one fell ill. A council investigation into the cause of the contamination could not pinpoint the source of the bacteria.
The Tonkin and Taylor report released yesterday said contamination may have come from stormwater sources, or neighbouring Te Mata Mushrooms. The farm completed earthworks which may have "compromised" the aquifer, the report said.
Wastewater from the mushroom farm, which may contain bacteria, could have become "hydraulically connected" to the groundwater system.
The report recommended the council assess whether the changes to the landscape could adversely affect groundwater, and that it do a "human health risk assessment".
It further recommended an expanded testing programme to better identify the source of contamination.
Te Mata Mushrooms owner Michael Whittaker said the report investigates an event three years ago and commissioned 18 months later.
"We are really disappointed it is being released in draft form, with people drawing conclusions in the heat of the moment," he said.
"In our view it doesn't draw conclusions."
Hastings Mayor Lawrence Yule also said the report was inconclusive but its findings would be included in a wider investigation of the current outbreak. Yesterday Health Minister Jonathan Coleman confirmed there would be a Government-initiated independent Inquiry into the contamination issue
A test on which animal the campylobacter came from is due today.