The New Zealand Ministry of Agriculture (MAF) says an American laboratory may have been misinterpreting tests for genetic modification in corn bound for New Zealand for up to two years.

Biogenetic Services - one of three companies accredited to test for GM in corn - had its licence suspended in March after an audit.

The ministry told the Local Government and Environment Select Committee that the audit found Biogenetic had been misinterpreting ambigious results as negative.

The director of plants biosecurity, Richard Ivess, said that the lab reported ambiguous indications of low-level contamination as negative results.

Under this country's zero tolerance policy those tests should have been reported as positive for genetic modification.

But Ivess says MAF cannot tell how soon the faulty interpretation began after the lab was first accredited in May 2002.

He says MAF is looking at requiring labs meet international standards to minimise mistakes.