A report commissioned for the Department of Environment, Climate Change and Water says the mistake was caused by inaccurate surveying.

The consultant's report recommends the department use better equipment and do more field work.

Department spokesman Tom Grosskopf says it is looking to introduce 3D imaging technology.

"The identification of old growth is highly variable and problematic and it identifies for us areas where we can improve our process," he said.

"We will be implementing all the recommendations in the report, to improve that process."

Carmel Flint, from the North East Forest Alliance, says the report vindicates claims the environmental group has been making for the last three years.

"It really confirms our worst fears about what's been happening, " she said.

"The review only looked at 12 sample properties, but based on its findings it looks like the Department have been approving about 75 per cent of mapped old-growth forest for logging."