Dutch police have found thousands of letters buried in holes in a forest by a postman, officials said, adding that they are seeking a motive.

Police made the discovery on Wednesday after a walker near the central town of Laren alerted them to mounds of earth and part of a shovel.

They found a "significant amount" of mail in eight holes each measuring one metre deep. Some were buried for weeks.

"They were mostly bills, magazines, and publicity letters," a spokesperson for the Sandd postal company said.

The company said it was not revealing the identity of the postman.

"It was someone who has unfortunately not respected our work standards. We are taking the necessary measures," the spokesperson added.

They did not comment on why the postman was believed to have buried the letters.

The letters that are still in good condition are being sent to their destinations, albeit several weeks late.

Those that have rotted will be destroyed after consultations with the senders.

"They've spent several weeks underground in a forest. They are mouldy and dirty," the spokesperson said.

"We very much regret what happened. Our goal is clearly to deliver mail on time. Clearly that didn't happen," they added.