A large number of these members earned low wages, many below the living wage, according to the union.
New parking fees were $1 per day for staff, but there were a number of exemptions.
Car poolers are able to park at the hospital for free, and the PSA says the DHB has agreed not to charge staff earning less than the living wage.
DHB Chief operating officer Sharon Mason said the new charges were put in place to free up more parking spaces.
"We believe that more parks will become readily available for our visitors who will be required to pay $1 for every 3.5 hours parked on site," she said.
The DHB decided to implement paid parking after receiving a large number of complaints from people struggling to find a park at the hospital.
Most regional DHBs, such as the Bay of Plenty and Whanganui, did not charge staff or visitors for parking at their hospitals.
Northland DHB introduced a charge for parking in 2015, due to a lack of car parks at Whangarei Hospital.
Staff at Whangarei Hospital were charged $1 a day, or $15 per week for a reserved park.
Visitors to Northland's main hospital were charged up to $8 a day, although the first hour was free.
Public hospitals in Auckland charged visitors and staff up to $18 a day for parking.
The PSA complained last year when staff parking at North Shore and Waitakere hospitals was set to rise from $600 to $720 a year.
The union took the opportunity to speak out against car parking fees for patients and visitors at the time.
"The PSA strongly believes any parking costs for patients and their family is unacceptable," a spokesman for the organisation said.