New Zealand's reputation as kiwifruit exporter could be badly damaged by news as many as 50 growers have been illegally injecting their crops with antibiotics in a bid to stop the spread of PSA bacteria, a toxicology professor says.

Zespri spokesman David Courtney said the exporter had tested every kiwifruit orchard in New Zealand and 1 per cent of the industry, all from the Te Puke region, had admitted illegally injecting their crops with the antibiotic streptomycin.

The industry is confident that none of the fruit made it to market after a rigorous testing system and is urging export markets not to be concerned about New Zealand's kiwifruit.

But University of Canterbury toxicology professor Ian Shaw told APNZ the news had the potential to do "huge" damage to the industry's reputation.

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"If we now taint our export markets with the potential for illegal chemicals being in kiwifruit we could destroy the industry altogether. I can't understand the rationale of people doing this, it just makes no sense," he said.

Some people were allergic to the antibiotics and could become seriously ill if they got a dose in their food - potentially a great deterrence to foreign buyers.

"If you spray really early on, before the fruit flowers form, by the time you get to the fruit stage all the antibiotic is gone.

"But the naughty farmers that are illegally using streptomycin are injecting it into the vines at a much later stage which means that the fruit will contain residues."

The illegal use of streptomycin also had the potential to create antibiotic-resistant bacteria, he said.

Kiwifruit growers in the region have been becoming increasingly desperate following the outbreak of the bacteria, first identified on New Zealand kiwifruit orchards in November 2010.

At last count, it has cost the industry nearly $900 million.

At the height of the PSA outbreak, growers were allowed under very strict conditions to spray a diluted form of streptomycin on vines before fruit formed but were never permitted to inject it.

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The Ministry of Primary Industries is investigating the practice.