A sustained years-long campaign involving underwear being placed in front of a tree outside a house in Melbourne's east has left police and the homeowner baffled.

The perplexed Surrey Hills resident has told law enforcement that knickers of various styles and sizes have been appearing in her front yard since 2014.

Someone has been leaving underwear at a tree out the front of someone's house.

At least five times, the woman has found a pair of undies carefully laid out in the same spot at the base of a tree trunk. On the sixth occasion, a black lacy number was put inside the woman's letterbox.

While mystery surrounds the origin of the multiplying smalls, police are unsure if an offence has even been committed.