But the man pointed the SPCA to the website of the Greater Wellington Regional Council (GWRC), which advised anyone who had live trapped possums to shoot them, drown them or take them to a vet to be euthanised.
Because of the misinformation, any prosecution was doomed to fail and the SPCA had to resort to formally warning the farmhand.
GWRC biosecurity manager Davor Bejakovich said the website advice had come from a brochure produced in 2003 that contained "best-practice information at the time".
"The information was removed and all the hard copies of the brochure destroyed when SPCA contacted council in March and pointed out that brochure contained erroneous information," he said.
The council's advice was now in line with the National Pest Control Agency's website, which said the most humane ways for killing captured animals were "shooting with a single shot to the head, a blow to the head using a hammer, bar, or stout wooden stick, or euthanasia by a veterinarian".
SPCA chief executive Ric Odom said the drowning of any mammal, whether it was considered a pest or not, was inhumane and could be deemed a criminal offence.
Though the GWRC was updating its website, Mr Odom warned that anyone aiding a person who caused an animal unnecessary suffering could also be committing a crime.
"So no one should ever drown animals and anyone advising people to do so may be liable under the act because they could be seen as 'counselling' other people to commit an offence," he said.
Drowning is not considered a humane method of euthanasia because of the suffering before loss of consciousness.
After submersion, Mr Odom said, the animal would panic and struggle to breathe.
"This will cause severe distress and pain that can continue for many minutes before the animal becomes unconscious and ultimately dies."
