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A passenger jet was descending about nine kilometres from the airport on Boxing Day when the pilot radioed ground control about his unlikely sighting, Fairfax Media reported.
It was identified as a remote-controlled, helium-filled shark - which has proved a popular Christmas gift this season.
Designed for indoor use, the $99 1.44-metre-long Air Swimmer can be operated by remote control over a range of 15m.
Airways spokeswoman Monica Davis said the pilot had reported the shark on December 26 but its altitude and how close it came to the aircraft were unclear.
Air traffic was alerted to the shark but no-one else reported seeing it, she said.
New Zealand Air Line Pilots' Association president Glen Kenny said the biggest risk the shark posed was startling the pilot.
It would probably not stop a jet engine if it was sucked in but could do a bit of damage, he said.