ecoli is from effluent. Whether that's from sheep (it's endemic in them, that's why Scouts aren't allowed to camp on fields where sheep have been in the last six weeks or so) or more likely cattle, or humans…..it's not good, put it that way.
Domestic water is generally chlorinated to kill it. Makes the water taste dreadful, but it stops the spread of the bacteria.
I think your only recourse is really to suss out where your water is coming from, and where the contamination occurs.
The house is really starting to come together, isn't it ?
Best of luck with it.
M
"To kill or inactivate E. coli 0157:H7, bring your water to a rolling boil for one minute (at elevations above 6,500 feet, boil for three minutes) Water should then be allowed to cool, stored in a clean sanitized container with a tight cover, and refrigerated. Currently, there is no filter certified to remove bacteria from water. "
From the CDC in America where well water is much more commonly used than here.
Bit of a beggar to do for every bath full or washing machine load though. I'd just bleach it at source. They do say that wells can be sterilised too though.