The Berejiklian government has developed secret plans to lift the height of Warragamba Dam more than publicly announced to take into account the impact of climate change, the Herald understands.

Officially, Infrastructure NSW has been working on a project that would raise the dam wall of Sydney's main reservoir by 14 metres to 144.5 metres, flooding as much as 5000 hectares of the Blue Mountains World Heritage Area.

The Warragamba Dam is proposed to be lifted by 14 metres but construction is understood to provide for a future raising to 17 metres. Credit:Edwina Pickles

However, a document seen by the Herald shows construction would involve building 17-metre-high abutments that would permit a later raising with minimal additional work.

The difference between the "operational raise" of 14 metres and the higher "structural" one was to cater for the impact of greater frequency of flooding because of climate change, it said.