An Australian traveller has received a $400 fine after four fruit flies were found on an undeclared mandarin.

Ministry for Primary Industries staff revealed today how the larvae were found earlier this month at Wellington Airport. The mandarin was being carried by an Australian passenger arriving from Melbourne.

The larvae have since been confirmed as Queensland fruit fly which is regarded as one of the worst horticultural pests in the world.

"One of our quarantine officers detected the fruit when the passenger's bag went through an MPI biosecurity x-ray machine. Another officer discovered insect damage on the mandarin and pulled the skin off, finding the larvae nestled inside," says Andrew Spelman, MPI border clearance manager, Central and South Region.

"Keeping fruit fly out of New Zealand is one of our highest priorities. The Wellington interception shows the value of MPI's biosecurity x-ray technology, which has been upgraded over the past few years."

So far this year, MPI staff have made 11 fruit fly interceptions at the border.

Spelman said MPI check all produce seized from arriving passengers for signs of pests or diseases. It also checks organic material disposed in MPI amnesty bins at airports and ports.