Wade Churchward, 28, went to a service station on March 22 in the capital, Wellington, where he picked up two packets of M&Ms candy and some potato chips, the Dominion Post newspaper reported.
Churchward, who had been drinking, began snacking on the goods while standing in line at the cash register.
But when he reached the cashier, he realized he did not have enough money, and instead offered a container with 0.042 ounces of marijuana and a pipe for smoking it.
Churchward failed to notice a patrol car parked outside and a police officer standing behind him in line - who promptly arrested him.
He pleaded guilty Monday in Masterton District Court to possessing marijuana and to several unrelated charges, the newspaper said.
Masterton police prosecutions section staff were unavailable Tuesday to confirm the details.
Churchward was released on bail and will be sentenced on July 3. Marijuana possession is usually punished by a minor fine in New Zealand.
