The Rural Fire Service is monitoring a rare underground fire beneath a lake bed to the west of Wilcannia.

Crews put the fire out several weeks ago, but it has come back this week.

RFS Superintendent Chris Favelle says he has considered digging the lake up to put out the blaze, but it is safer just to watch it and let it burn out.

He says he has never seen anything like it.

"What it is is a build-up of organic matter under the surface that's built up over thousands of years," he said.

"I'd say someone's lit a campfire on top of it, and it's just ignited under the ground, so there's enough oxygen under there in the soil to keep it smouldering away under there for as long as it wants to.

"A very difficult one from a firefighting perspective, I've got to tell you!"

Supt Favelle says it is creating a fair bit of smoke, but no people or properties are in danger.

"We could probably use heavy machinery to dig up the lake bed but that's not really feasible, particularly when you consider the heritage issues around the lake and so on," he said.

"So at this point in time all we can really do is keep a watching brief on it and try to ensure that it doesn't get off the lake bed itself."