Up to 1300 hectares of world heritage listed Blue Mountains bushland could be permanently damaged by floodwaters if the Warragamba Dam is raised, an internal document has revealed.

The draft environmental impact statement from September 2018, seen by the Herald, suggests the 14 metre increase to the dam would result in an "overall high direct (physical) impact" to the world famous area, which includes a number of Aboriginal cultural sites.

Views of Warragamba Dam, which the NSW government wants to raise by at least 14 metres. Credit:Wolter Peeters

The bushland lying within the proposed temporary inundation area would be "permanently" affected in the event of a significant flood, the document states.

It adds there were no proposed measures to avoid or reduce the level of impact to the area.