The cost of power of attorney has skyrocketed by as much as 250 percent, leading to fewer people signing the documents, the Law Society says.
An enduring power of attorney is a legal document that names the person who takes control of another's finances if they lose the capability to make those decisions themselves.
Parliament introduced changes in 2007 which meant an independent witness needed to be present when someone established an enduring power of attorney.
The Law Society said the change aimed to ensure people weren't signing the document without fully understanding what they were doing, but it had had other consequences.
Jonathan Orpin, a barrister and member of the Law Society's reform committee, said the rule had dramatically increased the cost.
He said in most cases where someone was thinking about setting up a power of attorney, a couple would go to see a lawyer and would appoint each other to look after themselves.
But now that had become more costly and a couple could expect to pay between $400 and $1000 for a power of attorney, he said.
"So in the past one lawyer could have made and sorted out the enduring power of attorney for both members of the couple, but that independence requirement means that the lawyer can only act for one of the two parties to the couple, and the other one will have to go and see another lawyer - which effectively doubles the number of lawyers involved, and that of course adds cost," he said.
"But in provincial New Zealand, it also means that there may not be enough lawyers to go around to do it, so people have to drive across the country, so people are being put off making the decision about who will look after them when they're well enough to do it."
Mr Orpin said the Law Society estimated there had been a 250 percent increase in the cost of processing the documents since the law change.
"Lots of people legitimately make the decision that, as the costs continue to rise, they'll put it off for another day and then another day, and then it becomes one of those things that never gets done and it's too late to do it."