Biosecurity officials say they can't explain the appearance of a single male Queensland fruit fly caught in Whangarei, since no produce is imported directly through the city's port.
Biosecurity officials on Saturday ended a two-week trapping programme without finding any more of the flies, which pose a major threat to New Zealand's $1.5 billion-a-year fruit export industry.
"We still don't know how it got here," Lesley Patston, a spokeswoman for the Ministry of Primary Industries. "It could have come from Australia or somewhere in the Pacific Islands."
The bactrocera tryoni fly found in Whangarei was only the fourth ever caught in New Zealand and in each instance there has been no sign that the destructive pest has got established.
In its native Australia, the fly is regarded as the most serious insect pest for fruit and vegetable crops including avocado, citrus, feijoa, grape, peppers, persimmon, pipfruit and stonefruit. Fruit is New Zealand's fifth largest export commodity.
Because the fly must have entered the country elsewhere, no changes would be made to procedures at Whangarei's port, Patston said.
Labour's agriculture spokesman Damien O'Connor said that the Queensland fruit fly is the horticultural equivalent of Foot and Mouth disease.