A formal warning has been issued to 26 kiwifruit growers for misusing an antibiotic spray in their fight against the Psa vine disease.

An investigation by the Ministry for Primary Industries (MPI) found a chemical spray called streptomycin had been used illegally on 45 different orchards.

MPI compliance director Dean Baigent said the ministry had been mindful throughout the investigation that New Zealand kiwifruit growers were facing a desperate situation with Psa.

"The growers concerned voluntarily disclosed their wrong-doing and those who misused product on a larger scale sustained the largest losses through the disposal of their affected crop."

A formal warning letter was an appropriate conclusion to the investigation and any future breaches of the Agricultural Compounds and Veterinary Medicines (ACVM) Act 1997 would result in prosecution, Baigent said.

Streptomycin could only be used on kiwifruit under strict conditions to avoid any possibility of chemical residues remaining on fruit.

Those conditions included a maximum of three spray treatments onto leaves prior to vine flowering.

Breaching the (ACVM) Act could result in fines of up to $150,000 for companies and imprisonment of up to two years for individuals.

The investigation's main purpose was to identify every case of misuse so all potentially affected fruit could be isolated and withheld from sale.

"In this we were successful and both domestic and international markets were able to be assured that no fruit with streptomycin residues was in the supply chain."

Kiwifruit marketing body Zespri was involved in the investigation, co-ordinating a declaration process for growers if they felt they had breached the conditions.

"There was a 100 per cent response from growers and those concerned have co-operated fully throughout the investigation," Baigent said.