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.