Farmers are "frustrated" by news that a big maize consignment has been contaminated by genetically modified (GM) material, endangering export markets.
Around 13,500 tonnes of maize -- all from one region -- may have to be dumped, after routine testing revealed the presence of GM contamination.
This is believed tobe the sixth such incident in the past three years.
Federated Farmers' spokesman Hugh Ritchie said while tests were supposed to pick up GM seed in imported seed, no test was 100 per cent accurate.
"There's only a 0.05 per cent of something getting through but the eventuality does exist," he told National Radio this morning.
He said it was "unlikely" that the incident was related to the last major contamination 13 months ago, when nearly 4000 tonnes of corn grown in Gisborne, Hawke's Bay, Waikato and Northland was found to have traces of GM material.
The problem facing growers was the maize industry relied on hybrid grain lines and it was necessary to continually import new genetic material, he said.
"We can't test every single seed because then we would have no seed to plant -- so we have to test small samples."
Ministry of Agriculture and Forestry (MAF) officials face a daunting task in tracking down the source of the contamination, as the seed came from a complex mix of seed lines and growers.
Mr Ritchie said "in hindsight", it would have worked better to test consignments from individual properties before mixing them up.
However, testing in terms of imports and seed planted was already "very rigorous" and had been strengthened since the last incident.
"You have to go with the processes in place and believe that that process is giving farmers protection," he said.
"But having said that it's very frustrating because farmers and seed producers are trying their utmost to provide seeds at zero tolerance."
He said it was hoped the seed would not have to be dumped, but its use would have to be "very carefully monitored" so as not to put any markets at risk.
MAF eradication programmes manager Ian Gear said it was expected to take at least three weeks to complete the investigation into the contamination because of the complexity of the situation.
