A BBC Wales inquiry has revealed that a water treatment plant at the centre of an stomach bug outbreak had already been told to raise its standards.
So far 144 people in north Wales have contracted the bug cryptosporidium.
The radio programme Llinyn Mesur has seen an independent report saying water from the Cwellyn reservoir works failed industry standards twice in 2004.
But Dŵr Cymru Welsh Water said there was no link between those incidents and the current outbreak.
Meanwhile, health officials overseeing the outbreak said on Sunday the number of people ill with the bug, whose symptoms include diarrhoea and stomach cramps, had risen by six to 144.
There are now two helplines in operation for people with concerns to contact.
Residents wishing to find out whether their home's supply is affected can check their postcode against a list held by Dwr Cymru Welsh Water at any time by ringing 0800 052 0130.
NHS Direct is operating two help lines on 0845 850 9850 and 0845 600 6086 between 0900 GMT and 1800 GMT on weekdays, and 0900 GMT and 1700 GMT at weekends.
Around 70,000 householders supplied by the Cwellyn reservoir near Caernarfon have been asked to either boil their water or drink bottled supplies until 9 January at the earliest.
The source of the cryptosporidium parasite has not been confirmed but the reservoir has been at the centre of investigations.
The current affairs programme Llinyn Mesur said it had been told that Welsh Water, the company responsible for the Cwellyn plant, had been under a legal obligation to raise standards there since it failed industry tests in February and December 2004.
A report published by the Drinking Water Inspectorate showed coliform bacteria present then.
In a statement , the inspectorate said Welsh Water was obliged to raise the standards of work in order to eliminate any problems at the plant.
But a Dŵr Cymru Welsh Water spokesman said: "The presence of coliforms at Llyn Cwellyn in 2004 does not suggest that there was any sewage contamination of the water supply, and we were not aware of any related illness in the community."
Only low levels of the bug have been found at the reservoir
Welsh Water had since agreed with the Drinking Water Inspectorate a programme of changes to further reduce the possibility of any statutory failures at the works - and that was under way, he added.
"Protection of public health is our highest priority," the spokesman said.
"The Cwellyn works is performing well and producing water of high standard.
"It is important that the outbreak control team can complete its work to discover the cause of the current situation."
The number of confirmed cases of cryptosporidium have risen steadily since the outbreak was announced on 24 November.
DNA tests are being carried out to discover the source of the bug, which is thought to have contaminated the reservoir via untreated human sewage.
The affected area includes Bangor, Caernarfon, Beaumaris, and Menai Bridge. Public health experts said the outbreak should "burn itself out", but if that has not happened by early January, the advice to boil water will be extended.
Dŵr Cymru Welsh Water has said customers might receive compensation for the inconvenience of having to boil their drinking water.