The Prime Minister, Julia Gillard, reignited debate by pledging $50 million to building up the dam wall by 23 metres.

Steve Knight, the executive engineer of the state government's dams safety committee, said without a higher dam wall, there was little scope for Warragamba to reduce downstream flooding in a big event.

''The full supply water level of the dam is very close to the top of the gates, compared with several metres lower at Wivenhoe Dam that provides flood mitigation space for Brisbane,'' he said.

However, Stuart Khan, a senior lecturer at the water research centre at the University of NSW, said while the risk was ''very real'' that the Hawkesbury Nepean Valley would flood, raising the height of Warragamba was the wrong way to deal with the issues.''It doesn't matter how big that dam is. It's not that it's not big enough; it's just that the management needs to change,'' he said. ''We need to reserve some storage capacity in the reservoir for when those big inflows come along.''

The Premier, Barry O'Farrell, was non-committal about the Prime Minister's suggestion for raising the wall of the dam because it was not backed with a sufficient financial commitment.

He declined to support the idea and would only say that the NSW government announced in December that it would review the major flood mitigation options in the Hawkesbury Nepean Valley.

He said it would look at ''minimising the potential economic and social impact of flooding within the catchment''.

The NSW Greens said the proposal was ''an expensive and ill-thought policy that fails to consider the cheaper, low-impact options''. The Greens MP John Kaye said ''at best it will be an expensive Band-Aid solution that will fail to effectively eliminate flood risk''.

While the region has not had a major flood since 1991, it remains a focus of insurers and emergency service planners alike. The area has recorded 120 floods in the past two centuries but the influx of many more residents from Sydney's sprawl has raised the economic and human risks of future floods. The damage bill from a major flood is estimated to reach as high as $8 billion.

The biggest flood recorded in the river's history took place in 1867, when water levels at Windsor peaked at 19.3 metres. However, raising the dam wall by 23 metres would only have reduced the impact of a flood on that scale by four to five metres, a report commissioned by the state government last year found.

The cost of raising the wall, put at $411 million last year, is likely to be much higher when construction is complete.

''It would be $700 million to $800 million,'' said Amir Deen, a water consultant and former senior hydrologist with Sydney Catchment Authority.

''There would be water quality issues for Sydney'' during construction, he said.

Apart from the direct financial cost, building up the wall would massively expand Lake Burragorang, which would back up along about 118 kilometres of remote rivers behind the dam.

It would inundate about 7500 hectares of protected bushland and spill into three adjoining national parks.

''It's a wild, wild area that's beautifully rugged, with deep ravines, some sweeping bends with rock pebbles and glassy water,'' said Keith Muir, of the Colong Foundation for Wilderness.

But, with the current dam, there remains a bigger danger of massive floods hitting thousands of homes downstream, said a former senior engineer at the dam, who declined to be named.

''It's got the potential for vast inflows,'' the engineer said. ''Keeping the dam partially lower would have a marginal effect on flooding. Those people sitting there are in danger as we speak,'' he said, noting the dam was full and more rain was on the way.