Lawyers and advocacy groups for injured people are demanding the Accident Compensation Corporation (ACC) be transparent about its computer model which predicts how long a client will use its services.
They said they were concerned that the model was based on getting people off ACC, instead of helping them.
ACC said it had been using the system, known as a survival analysis model, for three years.
It uses information provided by nearly 2 million claims it receives each year to predict which clients are likely to need help and how long they will need the services for.
ACC specialist lawyer Warren Forster, who is a member of several advocacy groups, including Acclaim Otago, said that was troubling.
He's concerned that private information from clients is being used to sort them into low and high risk categories which the model can then use to figure out when to stop helping them.
"The focus of the system once they get to a branch is on exiting the person within a timeframe and it appears that is the statistic that's used to measure performance in accordance with this model," he said.
"There's a real concern that people with the same injuries will be treated very differently, based on their risk scoring."
John Miller, who is also a specialist ACC lawyer, said ACC has been known to reduce services in the past and he was worried this was just another way of doing it.
He said having a computer predict whether a person needed help or not de-humanised them.
"I've seen too often, even with humans doing the [filing], suddenly cutting people off and leaving the poor injured person on the back foot," he said.
"I'm even more fearful of a computer system because you're actually depriving people of their income and their treatment at a time where they could be and probably are very vulnerable."
But the chief customer officer at ACC, Mike Tully, said that was not what the model was about.
He said it was designed to process claims more efficiently and improve the way people were supported.
Mr Tully said since ACC started using the model, the number of days it took to set up weekly compensation for clients' claims had dropped from 11 days to seven.
"It's a tool that enables our case managers to work out the priorities and needs of our clients and enables the organisation ... to respond far quicker than what we've been able to do previously," he said.
Mr Tully said because all the data was anonymized, there were no privacy breaches.
'Nonsense' to call system secret - Minister
The Minister of ACC, Michael Woodhouse, said since ACC was established it had been collecting data from clients to make informed decisions on its services.
He said it was nonsense to call the modelling system new or secret.
Mr Woodhouse said the system was not being used to reduce services to injured people.
"It doesn't predict it, it gives a guide to it because every situation is different," he said.
"But when you're dealing with nearly 2 million cases a year, trends do emerge and that's helpful for decision-making, it's helpful for deciding what levies we set."
ACC said it would post all the information about the model on its website, including all the variables it uses to make predictions, by early next week.