The ball was found close to the Ti Rakau Park, it said.
"There may be more of these around so please watch your dogs and kids around Ti Rakau park," the post said.
"This is a horrible thing to find, what type of person would take the time to do this do this, imagine the harm that could be done if a dog started chewing it or a kid picked this up. So please just watch out there could be more!"
The find has already been shared almost 3000 times, with supporters branding it "disgusting" and "horrible".
Among the flurry of disgusted comments, Kim Marie Miller said: "Geez! What weirdos in this world!"
Sharl Tate posted: "That's not very nice..must be painful for dogs and kids."
Another concerned poster, Laurie Hare, described it as "shocking" and said it was "just awful".
Sherryn Mehta also described it as "disgusting", adding: "Makes you ashamed to be a human. Crass, low life idiot."
While Emily Harris said: "Wtf! Some messed up people out there! Scum bags."
However, Ellie Fitzgerald, was one of several people who said it looked as though someone was using it as a needle cushion, rather than intending it for use as a weapon.
"First of all if you were messed up enough to leave 'weapons' around for children or animals to find wouldn't you leave it somewhere NOT in a gutter on the side of the road??," she asked.
"Secondly, why bother with little pins when you could just use nails causing a lot more damage? ... Also why would you go to the effort to place the pins all in a row? Some clearly don't even have the pointy end exposed!
"As a person who sews and does DIY, I can clearly see how someone has used this as a pin cushion, in fact, DIY painted tennis balls pin cushions are a thing!!
"It's insane how people are jumping to the worst possible conclusion."
