From Bearing Arms:

Chris Cahill, with the country’s Police Association, said it’s not clear if this year’s budgeted funding for the buy-back would be adequate because New Zealand (like America) does not have an official gun registry.
“We really have no idea how many of these firearms are out there in New Zealand,” Cahill told the Herald. “Which really points to how bad our firearms legislation has been, that we have let this get out of control.”