One of the people involved in the development of the federal government’s controversial aged care assistance tool says she’s now too scared to use it, saying she never wanted needs to be determined by algorithm.

As fellow advocates warned people’s care and funding needs were being underestimated, Lynda Henderson – who sat on the expert advisory group to develop the Integrated Assessment Tool (IAT) – said the assessment questions were aimed to assist those making clinical judgments.

But Henderson said she felt “fury” when she learned the government had introduced an algorithm to classify responses collected through the IAT to assign scores to responses and categorise people according to level of need. The resulting classification is used to determine the funding package allocated.

“I had no idea that an algorithm would eventually be applied to the assessment we were developing, which we began work on at the end of 2020,” Henderson said.

“My background is in psychology, psychometrics and statistics, and when we developed it, it was through the eyes of an assessor who would be carrying the assessment out, and so we made sure it would have room for nuance, notes and changes.”

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As previously reported by Guardian Australia, the IAT has been criticised after the government removed the ability for assessors to override an incorrect outcome when assessing people for home support. Aged care workers and clients say the algorithm frequently under-assesses levels of need, leaving people with inadequate funding and care.

Some people previously receiving home support packages – who required reassessment because their health deteriorated – said the IAT classified them at a lower level than before, leaving them with less funding and support despite greater needs, with assessors unable to override the outcome.

Henderson said members of the working group were sworn to confidentiality while developing the assessment, and believed it would continue to rely on an assessor’s judgment.

After the assessment questionnaire was handed to the government for trial, she said she felt proud of the work they had done, though she expected some refinements.

“But neither we in the working group or the health consultancy company that ran it knew the government were going to be using an algorithm to score each question and assign categories,” Henderson said.

“It is not the assessment or the questions that is the problem. It’s the absolutely ridiculously simplistic scoring algorithm that’s been applied to it.”

‘I am so terrified’

Henderson’s partner, Veda Meneghetti, died two years ago with dementia. Since then, her own health has declined.

Asked how she feels about the assessment she helped to develop now, Henderson said: “I really, really need more home support, and my case manager agrees.”

“But I am so terrified about applying for a reassessment and having all my support cut if it [the IAT] under-assesses me that I’m not going to go ahead with that assessment in the immediate future.

“I think that says a lot, really.”

She said she feels “white fury and frustration” about what the assessment has become. “The worst thing the government did was to disallow assessors to override it.”

The department did not respond to questions about when the decision was made to introduce the algorithm and who developed it. But Guardian Australia understands the rationale was to create a consistent, national approach to assessing care needs and reduce variation in outcomes. The information collected through the IAT is then used by the algorithm to determine eligibility and support levels.

Documents seen by Guardian Australia confirm the IAT became a prescriptive tool that could no longer be overridden when assigning support-at-home classifications on 1 November.

“An ongoing SaH [Support at Home] classification outcome cannot be overridden to an ongoing lower or higher SaH classification outcome,” the documents state.

Greens senator Penny Allman-Payne said the volume of complaints she has received about the IAT prompted her to write to the aged care minister, Sam Rae, on Wednesday urging him to “immediately reinstate human oversight of the IAT, and give assessors the ability to override under-assessments”.

“I have grave concerns about the use of algorithms and automation to make decisions about the complex individual care needs of older Australians,” she wrote.

“Older people are being denied urgent at-home supports, leaving them trapped in hospital or left without the necessary assistance they need to live safe and full lives in their homes.

“This is also placing an incredible strain on family members, many of whom are forced into unpaid care roles.”

The letter also raised transparency concerns, calling on Rae to make public details about who developed and tested the algorithm, and what methodology was used.

• Do you know more? melissa.davey@theguardian.com