An allegedly unsanctioned flight under the Auckland Harbour Bridge is being investigated by the Civil Aviation Authority.

The flight, captured by a person on a clip-on lane of the bridge, has been posted to social media.

It shows the plane travelling at speed and appearing to take-off from the water before it disappears under the bridge towards Birkenhead.

The flight happened on Boxing Day last year.

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An aviation authority spokeswoman said the footage had been brought to the attention of the authority and was now being investigated.

"We're treating it very seriously," said the spokeswoman.

Auckland Seaplanes, which operates off Wynyard Quarter have been approached for comment.