"Our greatest fear is that if the guilty party remains free, and not brought to account, that there will be nothing stopping them from doing it again to following years' crops - to us or any other orchardist," he said.
A second test around mid April picked up excess residue levels.
Mr Farrand said he had no doubts the kiwifruit were contaminated by someone who was qualified to spray Calypso.
"This has to be the work of someone who doesn't like me, to make me look like I don't know what I am doing. But I've been doing this for 36 years and have never come across a situation this bad," Mr Farrand said.
"A tractor hadn't been in there because in March my team picked and dropped off little gold kiwifruit on the ground which have not been disturbed. Therefore, whoever's been here did it with a knapsack spray. They know the industry, they know the system."
He has lodged a complaint with police, who he said needed a witness or someone who the guilty party had spoken to about the sabotage to come forward and give evidence.
Kerikeri police Senior Constable Robert Drummond said investigations were ongoing and he hoped the $50,000 reward would help bring in a successful arrest and conviction.
"We're dealing with a large amount of money he's lost as a result of this incident and we're obliged to pursue his complaint."
Mr Drummond said the most appropriate charge under the Crimes Act was contamination of food or crops, which carried a maximum penalty of 10 years imprisonment.
A Zespri spokeswoman said fruit from Mr Farrand's orchards failed multiple residue tests, recording many times over the allowed limit set by the government.
"We acknowledge the difficult circumstances for this grower."
Any information on those allegedly responsible for the contamination can be passed directly to kafarrand@gmail.com, or to Mr Drummond on 09 407 9211.