The NSW Greens say a public campaign echoing the successful rescue of Tasmanian rivers from damming in the 1980s is needed to preserve important Indigenous heritage sites and wild rivers on Sydney's doorstep.

The NSW government late on Wednesday secured support for a bill in Parliament that will allow the flooding of as much as 4700 hectares of World Heritage-listed Blue Mountains land.

The amended legislation was vital to allow the lifting of the dam wall of Sydney's main reservoir at Warragamba by 14 metres to help reduce flood risks in the Hawkesbury-Nepean rivers downstream.

Warragamba Dam wall: The government has secured passage of a key bill, opening to way for it to be raised. Credit:Brendan Esposito

"The government has made clear its intention to do this," Justin Field, Greens urban water spokesman, said, adding that opponents may have no choice but to take direct action. "The only way to oppose it is to stand in front of it."