Osborne resident Duncan Eddy said he was a regular visitor to the tourist spot because his 16-month-old son Kahu loved to chase the hens and roosters in the car park.
However, on a visit last weekend he was disturbed to find two of the hens dead, a third on its back, flapping its wings weakly, and a fourth hen appearing "drunk" and struggling to walk.
A visit by the Otago Daily Times to the site yesterday found one dead hen and no sign of any living hens.
"It's sad. They're an institution," Mr Eddy said.
"It's not the best entertainment, taking your kids to visit the roosters and finding them dead or dying."
Mr Eddy said numbers in the brood had been growing recently, and on his last count a week ago, there were about 20.
He believed the hens had been poisoned.
"Maybe someone thought they were pests because there has been a lot of them around there recently."
Dunedin City Council Environmental Health team leader Ros MacGill said the council had not been using poison in the area, and staff normally relocated problem hens or roosters rather than poisoning them.
A pest control contractor had been at the site on Monday and saw dogs chasing the brood.
"That may be the cause. But I can't rule out the possibility that someone else may have taken the matter into their own hands."
The council would be keeping a close eye on the situation, she said.
Mr Eddy said if someone was laying poison in a public place, they should consider the small children running about the area.
"It should be well signposted."
