Legitimate hunters in Hwange, Zimbabwe's largest national park, have taken aerial photos that show the reserve littered with fallen and deflated elephants numbering more than 300.
"This is the largest massacre of elephant in this part of the world for the last 25 years," said Tom Milliken, programme leader for the Elephant and Rhino Traffic network.
Zimbabwe has one of Africa's largest elephant populations, and about half the country's estimated 80,000 live in Hwange.
The poachers have been poisoning watering holes and salt licks, killing not just elephants, but any animals using the watering hole or scavengers feeding on the elephants themselves.
Caroline Washaya-Moyo, a spokesman for Zimbabwe's National Parks, said 10 more poisoned elephants were found by authorities last week.
She said she was "surprised" by the hunters' report, but admitted that parks officials only began their own aerial survey last week. "We did find that (searching for carcasses) is more efficient from the air," she said.
So far, four poachers have been jailed. Authorities say the cyanide has been planted by villagers who then sell the tusks for less than $500 each to traders across the border, who can then more than $15,000 per pair in South Africa.
Authorities have given villagers surrounding the park until the end of October to turn over any cyanide, but local leaders are saying the villagers were driven by poverty, while the real traffickers profiting from the ivory are getting away.