Former senior officials and engineers at Northern Ireland Water say management mistakes helped exacerbate the 'Big Freeze' crisis of last Christmas and New Year.
The ex-staff also warned that another big freeze could again see tens of thousands of people without water if changes were not made to the way NIW is run.
Private correspondence seen by the Belfast Telegraph reveals internal alarm bells were being sounded over a falling level of inspections being carried out on plumbing in properties.
The whistleblowing ex-staff, who asked not to be identified, say they were also flagging up claims that NI Water was failing to meet its statutory obligations in the two years running up to the calamity of last winter.
Around 40,000 people were left struggling without water over the 2010/2011 Christmas holiday period in about 80 towns and villages across Northern Ireland - some of them for up to eight days.
Chief executive, Laurence MacKenzie, resigned and a report by the Consumer Council said NIW had been guilty of failings "on a monumental scale".
Now engineers made redundant over the past two years insist their warnings should have been heeded - and that "mismanagement and maladministration" have left NIW in no position to cope with a repeat of the crisis.
One former plumbing inspector said: "We were always conscious as was the company that there were legal obligations, but I believe their failure to fulfill their statutory obligation to carry out plumbing inspections (under Article 114 of the Water and Sewerage Services NI Order of 2006) was a direct and a very significant contributory factor for the hardship suffered by people during the freeze."
A letter sent to a senior NIW manager in June, 2009, argued: "...vacancies mean that judgements (and therefore risks) are being taken in relation to the preliminary surveys for water connection applications, and [that] there are insufficient resources to perform the inspection of internal plumbing before properties are connected to the water supply system, exposing NIW to a breach of our statutory obligations...."
It also said there had already been three cases in which pressure boosts or extended water mains were required because the failure to carry out inspections meant insufficient water pressure had not been identified, and described the incidents as "costly mistakes which could have been avoided..."
Then in September 2009, a memo again sent to senior management reiterated: "I know through our discussions you are aware of the issues but it's worth repeating that NIW are currently not meeting their statutory obligations with respect to this activity."
Another letter seen by the Telegraph warns: "Given the same weather conditions next winter, tens of thousands of N I Citizens will again find themselves without water and suffering great hardship.
"I believe this arrogant refusal to learn from mistakes shows a critical weakness within the mindset of NI Water management and a lack of understanding of operational procedures and lack of professional direction at a senior level."
A former senior officer also added: "In 2007 N I Water were so keen to shed staff they went too fast too soon.
"At the same time they paid off experienced water engineers and employed individuals from the private sector and other non-water utilities."
In reply NI Water said new regulations have replaced the former legal obligation and the current position makes householders and those who install plumbing systems responsible.
"The new Regulations revoked The Water Regulations (Northern Ireland) 1991," a statement said.
"The new Regulations place a legal obligation on owners and occupiers of premises and anyone who installs/alters plumbing systems or water fittings to ensure compliance with the Regulations."
A spokesman added: "NI Water has a dedicated inspection team trained to a level comparable with other water companies."
A highly-critical Consumer Council report said the major incident plan by NI Water to deal with the Christmas freeze-up was "neither adequate or fit for purpose".
And, it concluded, the problems of water-starved customers were made worse by a lack of sufficient and correct information.
Around 40,000 people were left off supply in an escalating disaster which lead to the resignation of chief executive Laurence McKenzie.