Mr Blair said the homemade dart appeared to be made out of plastic from a soft drink bottle, black PVC insulation tape with a nail fixed to the end.
Mr Blair said there were concerns there could be more darts like it out there and potentially more cases of animal cruelty, or worse.
Mr Blair said the duck would have likely died a slow and painful death if it had not been found.
"If the duck was not rescued complications from infection, and swelling could have resulted in the duck not being able to eat or in fact breathe . . ."
The a lot of time and effort appeared to have been made in creating the dart, as if it was made for a purpose, he said.
"The dart looks to be air propelled. If it was blown by mouth the person would have to have been close to the duck to hit it. It is also possible that a compressed air device was used."
Mr Blair said it was hard to know if the shooting of the duck was mindless fun gone wrong or something more sinister.
Either way, the SPCA was keen to hear from anyone who saw suspicious activity in the reserve near Bermuda Drive on Sunday.
"It's concerning but it's not foreign to us. We get called to an increasing number of these things where people are getting fun out of harming an animal without any regard to what harm they are doing or what pain they are putting the animal through."
The duck was now recovering at the Oropi Native Bird Rescue Haven.
Information can be passed to Mr Blair at 5780245 ext 705 or inspector@taurangaspca.co.nz