Audit NZ also found Wellington Water was unable to accurately report the number of complaints for wastewater, stormwater and water supply services.
"Complete records of all complaints were not available, and the complaints system used also did not classify complaints between wastewater, water supply, and stormwater", the opinion said.
Furthermore, Wellington Water couldn't accurately report the number of dry weather sewerage overflows.
"The system used for recording events included blockages in the wastewater network that did not necessarily result in an overflow."
In the past year there were 2096 of these spills across Wellington Water's network. The target is less than 100.
The company noted that regardless of the finer details of the methodology, there would still be hundreds of these events with or without blockages being included.
In a statement to the Herald Wellington Water acting chief executive Tonia Haskell said Audit NZ was not questioning the company's performance or results, but rather its ability to independently verify their accuracy.
"Wellington Water has a strong focus on delivering customer satisfaction and meeting our client and council expectations.
"We accept Audit NZ's position, and are working on how we record and manage our work."
Remedial actions include staff training, improved data collection, making some photographic evidence mandatory, working with councils to improve recording of complaints, and routine quality assurance checks.
Audit Director Karen Young said the company did have capability to resolve the issues by next year and would be seeking some external assistance.
Additional water meters would also need to be installed within the network to achieve this, she said.
Wellington Water also reported it's customer satisfaction survey was not robust enough in terms of design, sample size, and controls to produce reliable results.
The company said it would engage an independent survey firm to undertake the task from January 2021.
Audit New Zealand has previously raised concerns the company's non-financial measurement systems were inadequate, specifically around response times on callouts.
Upper Hutt Mayor Wayne Guppy has claimed Wellington Water wasn't transparent about those concerns.
He wrote to the Wellington Water Committee expressing his council's dissatisfaction around Wellington Water's performance measures, delivery and a lack of transparency and communication.
"We were concerned directors were writing letters to the auditor and the shareholders had not been informed."
But committee chairman David Bassett later confirmed he has been kept informed of the issue and has communicated relating correspondence.
Guppy has clearly been unhappy with Wellington Water, even going to the extent of criticising the language in the company's latest update report.
"There's a lot of flowery language in here that really doesn't tell us anything - 'the company is in good heart', well what the hell does that mean?"
He said the committee "needs facts" and if Wellington Water said in a report that something was going to save them money, it also needed to say how much.
