A wartime letter of condolence from King George VI has been found on an Auckland motorway.

The letter was in a box of photos, records and other family memorabilia found by a maintenance crew on State Highway 16 westbound just before Rosebank Rd on Sunday, February 3.

"The box was found in a live lane so must have dropped from a passing vehicle that had either come from the city centre or through the Waterview Tunnel," said NZ Transport Agency Auckland operations manager Rua Pani.

The agency is hoping to reunite the box with its owner.

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"It's a brown box with gold inlay. Fortunately, it didn't spring open and have the contents blown away by the wind," said Pani.

"So far, our efforts to track down the owner of the box have been unsuccessful, but we do want this story to end happily."