A conservation group is calling on the West Australian Government to change the criteria for classifying old growth forest after it found karri trees up to 600 years old are being wood-chipped.

The Forest Alliance sent two karri samples - one from a stump in a clear-fell area and the other from a woodchip mill - to a laboratory in New Zealand for testing.

Alliance spokeswoman Jess Beckerling says the results are astounding.

"We've gone to the best radio carbon dating laboratory that is available and we've gone to the head of that school and he's given us a 91.4 per cent probability that the sample that we sent from the woodchip mill is between 511 and 596 years old," Ms Beckerling said.

"These 600 year old trees - ancient trees - are trees that are our natural heritage and for them to be getting torn down predominantly for woodchips, and those woodchips are being exported to Japan and coming back to us as junk mail brochures, is shocking to everybody in Western Australia, I'm sure."

Environment Minister Bill Marmion has been contacted for comment.