A gigantic chicken egg laid with a smaller egg inside it has baffled a poultry expert, who says he has never seen the phenomenon before.

The babushka egg was laid by a free-range chicken at the Stockman's Eggs on the Atherton Tablelands, west of Cairns, in far north Queensland.

Farmer Scott Stockman said one of the staff found the egg and he was there soon after.

"It was the largest egg that's ever been produced on this farm since it was started in 1923 — three generations ago."

Mr Stockman said the egg weighed 176 grams, more than three times the size of an average egg.

And as for how the chicken is faring, Mr Stockman said "she would have had a pain in the bum".

When it was cracked, they discovered a second smaller egg sitting inside.

"It's just incredible actually —to have two perfectly formed eggs together," Mr Stockman said.

Associate Professor Raf Freire from Charles Sturt University's veterinary sciences school said he had never seen anything like it before.

"To be honest, I don't really know how it's come about," he said.

One of these eggs is not like the others. ( Supplied: Stockman's Eggs )

Professor Freire said it looked like the chicken had produced an egg as normal, but for some reason it was never laid.

"It's not too unusual if a hen is very stressed, you sometimes see it in cages, where they retain that egg.

The massive egg with another inside would still be fine to eat, says expert Raf Freire. ( Supplied: Stockman's Eggs )

"They don't lay it, but behaviour just stops and the hen usually goes back to feeding."

He said the egg would typically then just fall out of the hen a few hours later, but in this case maybe that did not happen.

"Then the next day, rather than that egg being laid, like it usually is, what's happened is that there's been another ovum released."

"That's come down and then the chicken has somehow decided to make its shell around both the previous day's egg and the new ovum that's come down.

"Biologically I'm struggling to understand why that smaller egg never dropped out, it's very odd."

He said despite the odd occurrence, the egg should still be safe to eat.

"Normally the double yolks or the triple yolks are perfectly fine, so I don't see why there would be anything wrong with this one."

Mr Stockman said the egg came from a free range hen that runs around the paddock.

"We get about 50,000 eggs a day so we didn't bother eating it" he said.