A MAJOR flood of the Hawkesbury Nepean Valley in western Sydney would be more devastating than the Brisbane floods of 2011 and was just as likely to occur, said an expert witness to the Queensland government's post-flood inquiry.
The potential for a catastrophic flood in Sydney's west was placed on the agenda last year by the O'Farrell government's adviser Infrastructure NSW, which suggested raising the level of Warragamba Dam to prevent a future disaster.
And while the present rainfall in parts of NSW and Queensland is not expected to cause large problems in Sydney, the renewed focus on flood mitigation has heightened concern about what would happen if rain concentrated in the city's west.
''There is huge flood risk - it is probably greater than Brisbane,'' said a dam expert, Mark Babister, a witness in the Queensland Flood Commission's inquiry into the 2011 Brisbane flood.
''It probably would impact on more people, homes and small businesses,'' Mr Babister, the managing director of WMAwater, said. ''But it wouldn't have some of the economic consequences of shutting down a CBD.''
The most common suggestion to mitigate the risk of flood in the Hawkesbury-Nepean area is to raise the height of the wall at Warragamba Dam.
Lifting the height of the dam wall by 23 metres, which was suggested in a 1995 environmental impact study, would lower the flood level at Windsor Bridge by about five metres in the event of a one-in-200-year flood.
But in 1995 the Carr government instead decided to manage flood risk in western Sydney by upgrading roads to make evacuation easier as well as making the dam safer by building a spillway to release excess water.
Infrastructure NSW, in its report to the government last year, slammed this decision.
It said the choice of managing the ''issue through evacuation and planning has had either limited impact or benefit, or has been completely inadequate in reducing the social and economic impacts of flooding in the HNV [Hawkesbury Nepean Valley]''.
Another expert, Drew Bewsher, said the renewed focus on water management represented a opportunity to look at raising the dam wall again.
''You can have two or three decades without much flooding and people forget about flooding altogether,'' Mr Bewsher said. ''Now is the time to do something.''
The topography of the Hawkesbury Nepean Valley makes it uniquely susceptible to flooding, even if there has not been a major event since 1990.
The O'Farrell government has said it would conduct a ''strategic review'' of mitigation options for the area, but would not put a time on the review.