The Accident Compensation Corporation (ACC) is making $4 million a year running the car park at Auckland's Middlemore Hospital.
Parking fees at the hospital are under fire after the father of a premature baby started a petition calling for the $20 fee for four or more hours to be scrapped.
Alex Dexter spent hundreds of dollars on parking while visiting his son, and his petition now has almost 16,000 signatures.
Counties Manukau District Health Board owns the car park land but said it did not receive any financial benefit.
ACC held the parking concession contract and said the revenue helped cover the costs of its clients' treatment and rehabilitation.
The car park at Middlemore Hospital, in south Auckland, is the only car park ACC runs.
The DHB's hospital services director, Phillip Balmer, said ACC provided free parking to all haematology and dialysis patients and also provided free or concession parking to other visitors on a "needs basis".
In February, 960 free passes were given to parents or other caregivers of patients at the children's hospital, he said.
Another 107 concession parking passes were given to other visitors.
"We promote these options to patients and regret in this situation the father concerned was not informed," Mr Balmer said.