Alan Gordon injured his back and suffers from chronic pain; he is appealing an ACC decision to cut his entitlements. (Video first published December 15, 2018)

Tens of thousands of "blank template" letters have been left out of ACC's official reporting on how many claims it approves or declines.

Barrister Warren Forster, who has represented hundreds of ACC claimants, said in his experience the "CM03" documents were like "blank cheque" letters, often used by ACC staff to decline claims for cover and entitlements.

ACC said the letters could not be retrieved as it "would require ACC to undertake an extensive manual search through all claim files", but the agency was confident only a small number were used to decline claims.

Forster said it was "disturbing" a large government agency had such a significant gap in data, when that information was used for statistical modelling to manage claims.

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The annual number of claims declined by ACC has been disputed by Forster for many years.

In a 2015 report funded by the Law Foundation, Forster said the number of claims turned down by ACC could be as high as 300,000.

ROSS GIBLIN/STUFF Barrister Warren Forster, who has represented hundreds of ACC claimants, says the number of claims declined by the agency are much higher than officially reported.

ACC revised the number up from 70,000 to 90,000 in 2017 following a Government-funded independent review of ACC's dispute resolution services by Miriam Dean QC.

The figure increased again this year, with ACC advising Stuff the annual number of claims it had declined or found against was 99,500.

ACC said the figure was based on the number of standard decline letters extracted from its electronic claims management system (Eos) and moved to a data warehouse for reporting purposes.

It was not possible to search Eos for individual claim records, which may include decline decisions issued with CM03 letters, ACC said.

On Friday, an ACC spokesman said sampling of more recent CM03 letters showed "very limited use" for decline decisions.

In the first quarter of the 2017-18 year, 17,561 CM03 letters were generated, of which only 312 used words associated with decline decisions in the letter's description, he said.

In the first quarter of the 2018-19 year, 11,464 CM03 letters were generated, of which 176 used words associated with decline decisions.

ACC says only a small number of "blank cheque" letters are used to decline claims, but it cannot confirm the total number.

Forster said ACC's figures could be "wildly inaccurate" without counting all of the CM03 letters.

In an official information response to advocacy group Acclaim Otago in August, ACC said 125,698 CM03 letters were generated in its case management system in the 2009-10 year and 83,826 letters were generated in the 2016-17 year.

ACC could not say if those letters were actually sent, but was confident only a small number were used to decline claims.

"We have undertaken a high level search and are confident that only a small portion of the CM03 letters relate to decline decisions."

Staff had been instructed not to use CM03 letter templates to decline claims, the ACC response said.

"Since ACC revised and reduced the number of letter templates in its system, the overall number of CM03 letters generated has also reduced."

ACC told Stuff it provided the Minister for ACC with information on claims data, but not information about CM03 letters.

Forster said that was extremely concerning.

"If they are telling the minister they are only declining 99,500 claims a year then what they are doing is lying to the minister and they need to stop doing that."

ACC's instruction to staff to stop using the CM03 letters for decline decisions showed they had previously been using them for that purpose, he said.

The information gap was disturbing because that data was used for statistical modelling by the organisation, which was in turn used to set performance measures, Forster said.

"The input data is based on the 99,500 declined claims. If there are tens of thousands more decisions kicking around in ACC's system clearly the statistical models will be inaccurate."