From July 1, legislation came into force under which ACC deducts a 15% withholding tax from its payments to ACC clients or their carers.
The legislation treats non-agency carers of ACC clients as self-employed independent contractors.
Mr Miller, highly experienced in ACC cases, said families had to cope with the "administrative nightmare caused by this taxation amendment".
The changes, which seemed "so unnecessary", affected families looking after injured children, siblings and partners.
"It effectively means that many of them have suffered dramatic loss in the amount paid for attendant care.
"One wonders whether there is a hidden agenda to drive families out of caring for injured people and putting it in the hands of private agencies."
That outcome would be "disastrous," he warned.
The vast majority of families he dealt with provided wonderful care to the injured person, supported by ACC funding, he said.
"Agencies would be unable to replicate such a level of loving care."
Given the amount of money available, care agencies would in most cases only be able to hire low-level staff who did not have the interests of the injured person as much at heart as families, he said.
Denise Powell, the president of Acclaim Otago, an ACC support group, said last week she was shocked at the likely multimillion-dollar rise in ACC costs that would result from increased agency care.
Dunedin residents Elwyn and Vincent Kaye have established a company to enable them to continue their care for their son, John, a University of Otago graduate and ventilator-dependent tetraplegic.
The couple were strongly critical of the tax changes, which they say would mean higher costs for ACC clients, through legal and accounting fees, if companies had to be established.
Mr Miller said he had experienced "most frustrating" delays in trying to get some written information from the IRD in order to offer advice to clients.
Information about the changes which had been earlier sent out by ACC had been presented in a way which caused "considerable anxiety and consternation".
Practical solutions, such as arranging a company structure, should have been offered to enable at least the same amount of care to be maintained, Mr Miller said.
ACC lead media adviser Stephanie Julian said ACC and Inland Revenue ran an intensive campaign over the three months before the changes came into effect to inform caregivers and injured clients about their tax obligations involving payments for ACC-funded attendant care services.
She said the campaign included repeated mailouts, phone contact with caregivers and injured clients, and seminars.
David Udy, IRD group manager for assistance, said extensive efforts had been made to communicate about the tax changes.
The changes would generally make it easier for people to meet their tax obligations and he believed no administrative nightmare was involved.
Carers and ACC clients involved with agency care would experience no change.
Some other recent changes would also prove beneficial, Mr Udy said, including a favourable rise in tax thresholds.
