What came first: the possum or the egg?

It is probably not the idiom you are familiar with, but this is no ordinary marsupial.

Darwin resident Kathy Allen's furry friend has become a certified "chook nanny", proving anything is "possumble".

"For the last three weeks it [the possum] moved its way into our chook pen, and the chooks seem to be pretty okay with it," Ms Allen said.

"I had a peek inside this morning and there was a chicken on top of the possum laying the egg.

The furry visitor lies on the eggs without breaking them. ( Supplied: Kathy Allen )

"They seem to snuggle up together and the possum lays on top of the warm eggs."

Despite its best efforts — and to the surprise of no-one but it — it is in fact a possum, not a chicken.

It is not known what brought about the identity crisis.

Ms Allen said her clucky visitor had grown accustomed to his new lifestyle — and assures there's been no fowl play.

"Since the possum has become so comfortable, there's been no broken eggs," she said.

"They seemed to have worked things out."