Some clients and carers were angry about the changes, which had not been initiated by the ACC, Mrs Powell said.
Tomorrow, legislation will come into force through which ACC will deduct a 15% withholding tax from payments it makes to ACC clients or their carers.
The legislation will also treat carers employed by ACC clients as self-employed independent contractors.
If it is their full-time job, those carers are also likely to have to register for, and pay, GST.
The law has been changed, at least in part, to ensure carers are not evading tax payments.
One alternative offered by ACC is for carers to become employees of agencies offering in-home care.
Some families are concerned that the changes would either force them to accept a 12.5% cut to their support payments, in the form of GST payments, or risk losing control of the care of their injured family member, through family carers becoming agency employees.
ACC has said that ACC clients could also set up limited liability companies to employ their care-givers and manage the payment of GST.
This approach would result in GST being paid on top of the ACC rate paid for care-giving services, provided the correct paperwork was filed.
The agencies already provide care for 45% of the 3000 seriously injured people requiring extensive attendant care at home through ACC throughout the country.
ACC officials said that non-agency carers providing home-based care were usually paid about $14-$18 an hour, with agencies being paid about twice this rate.
Mrs Powell, who is the president of Acclaim Otago, an ACC support organisation, said that many ACC clients who could not face the burden of establishing a limited liability company themselves could switch their care to agencies.
This would roughly double the cost to ACC and eventually require higher ACC levies, she said.
Huge sums were involved, given that some ACC clients required 24-hour care.
There could also be other "serious consequences", which could result in extra stress for some ACC clients and in some cases "people's care could be compromised", she said.
Some clients had also had trouble finding out about the changes and confusion had resulted, she said.
Carers New Zealand chairwoman Jan Moss, of Auckland, said the extra bureaucracy could make it much harder for ACC clients recovering at home after an injury to find part-time carers to help with a few hours of vacuuming, food preparation or child care for a few weeks.
Previously, the client could have paid the carer small sums directly, but now a tax form would have to be completed, Mrs Moss said.
Laurie Edwards, the ACC corporate affairs manager, said ACC and the IRD had made considerable efforts over the past few months to communicate with affected clients over the changes, including through some phone calls from ACC case managers.
In some cases, clients and carers might need to also seek advice from the IRD, Mr Edwards said.
The main impact was increased paperwork, and this could be dealt with in several ways, including by, in some cases, asking agencies to provide care, he said.
