A Whangarei woman says she "feels grubby" after ACC hired a private detective to watch her and then used the information to reduce payments to her.
The woman, Karin Garry, of Whau Valley, has lodged a complaint through the Privacy Commissioner over ACC "snooping" which she says was an invasion of her privacy.
However, ACC says $731,758 spent in New Zealand on private investigators in 2013/14 saved $22.7 million in 2013/14, including an estimated $1.675 million in Northland.
Northlanders make up 3.33 per cent of the national population, but accounted for 41 - 16.9 per cent - of the 242 New Zealanders that ACC legally spied on in 2013/14.
Following an Official Information Act request, ACC also told the Northern Advocate that there were 230 clients (23 from Northland) investigated nationally in 2012/13 and 321 (13 from Northland) in 2011/12.
ACC spent $731,758 on private investigators in 2013/14, $542,266 in 2012/13 and $748,639 in 2011/12.
ACC senior media advisor Stephanie Melville said "allegations of fraud are taken very seriously and all reported cases, including public tip-offs, are investigated thoroughly".
Ms Garry, 58, fell heavily on to her right knee while walking her dog a year ago. She was a sickness beneficiary at the time and entitled to home help and medical assistance.
She told ACC she could not walk any distance, and required crutches and home help. However, an "investigation report" the corporation provided to the Northern Advocate says information was received that Ms Garry was misrepresenting her level of capacity and a private investigator was hired to undertake inquiries.
While under observation, Ms Garry was seen not using crutches, driving alone and was twice seen bending to pick up firewood she loaded into a barrow and wheeled into a shed. As a result, ACC called Ms Garry in for an interview on June 5 and her home help was reduced from 6.5 to 3 hours a week from June 9 to July 6, after which it was recommended to stop.
Ms Garry acknowledged ACC's findings through the investigator, but said she had recorded on her ACC file that she didn't use her crutches all the time because she found them difficult to manipulate.
"As for bending to pick up a couple of bits of firewood - that didn't involve using my injured leg."
Ms Garry said the private eye surveillance had her "looking over my shoulder" in the supermarket and no longer popping out in her nightie to get clothes off the line in her enclosed back yard.
"I feel soiled," she said.
Ms Melville said private investigators were not used in every investigation and over the past few years there had been fewer than four complaints about private investigators acting for ACC.