The fragments of carrot bait were killing birds as well, including birds that mainly eat insects. He was appalled, and leaked the information to newspaper journalist Anthony Hubbard.
When the story came out, in combination with some unintended cattle poisonings, there was an uproar, Peter's wife Ella remembers.
"We were among the very first anti-1080 activists," Ella Grant said.
The way 1080 was used to kill mammals was changed and refined, and the Grants talked to people they respected and found out more about it. Eventually carrot bait stopped being used and cereal baits were developed.
"We found that if it's done properly the birds are far better off. If there's some bird kill the population has bounced back to the same level in a year and increased in two years."
Fast forward to 2018 and Peter Grant, a veterinary scientist, is still a keen conservationist and founder of the Whanganui Science Forum. He and Ella own a 230ha block of bush and protect it for kiwi, falcons, whiteheads, robins and other birds.
When the Grants were asked whether 1080 poison could be dropped there to kill possums and control the spread of TB, they were happy to agree.
"Personally I'm pretty impressed with how much they can achieve with a very little amount of 1080," Peter Grant said.
He couldn't detect any lessening of bird numbers after the drops. Stories about people's feet crunching on the bones of dead birds after a 1080 drop are preposterous, he said.
The Grants fear anti-1080 campaigners will stop the drops happening - as happened recently in the Hunua Ranges near Auckland.
"Our biggest fear is that this hostility will leave all these birds and other animals stranded high and dry, because there's nothing else at the moment that's preventing them from going extinct," Peter Grant said.
The two are planning a science talk about 1080 for Whanganui, and said it may get heated given the level of 1080 angst at the moment.
They have turned a social media post by Daniel Hunt and a letter to the Whanganui Chronicle into an A3-sized 1080 information pack, and made it available to forum members. About 20 people have taken it for distribution. A few are dismayed by use of 1080 and one said it was the effect on deer that bothered him.
"For me, personally, if these magnificent animals were to disappear from our forests [the forests] would lose all interest for me," he said.
Opposition to 1080 drops has intensified in the last few weeks, with a hikoi that finished with the dumping of dead birds on the steps of Parliament on September 8.
There was a protest in Whanganui as the hikoi passed through on August 11, and some Whanganui 1080 opponents were at Parliament at its conclusion.
Operation Ban 1080, a Facebook page, now has 85,000 followers. People are systematically swamping live social media feeds with anti-1080 messages.
There have also been threats to "take down" helicopters delivering 1080 baits, and Conservation Department staff have been abused, threatened and had wheel nuts on their vehicles loosened, and tyres slashed.
Staff at Whanganui's Orillion complex, which makes the baits, have had death threats.
Whanganui's Tanea Tangaroa was at Parliament to protest on September 8, but said violence and threats were not for her.
"That's not us. I'm not part of any of that. My thing is about processes and keeping to that within Whanganui," she said.
Veteran protester Denise Lockett said those making threats were ruining the movement's credibility.
"I understand them protesting, but I'm against extreme measures of any kind."