Water NSW, responsible for overseeing operations at Wyangala and its proposed expansion, would this week not be drawn on projected costs, saying they would be included in the final business case, expected later this year.

The cost estimated in DPIE documents would be to physically extend the wall and does not take into account damage to downstream water users nor the economy during its estimated four-year construction.

Extending the dam wall by 10m would result in about 21 gigalitres more water being available in an 'average' year.

An independent engineer's report - lodged last Monday with a current state government upper house inquiry into water infrastructure - details projected operations needed to enlarge the dam.

Report author, civil engineer Patrick Brown, has previously been part of Melbourne Water's infrastructure management team, and has worked on issues related to Greenvale and Tarago reservoirs, both embankment dams, as is Wyangala.

He has also worked on Tasmania's hydro-electricity dam Lake Binney.

Mr Brown said his report dealt with issues "typical" of maintenance or modification of dams.

It hones in on the amount of rock fill needed to raise the wall - 1.4 million cubic metres, increasing the total volume of the embankment by 40 per cent - and from where it could be sourced.

Rock fill for the current wall was sourced within a kilometre of the existing structure, but those quarries and access roads are now under water. To access those quarries again would require reducing Wyangala's capacity to 45 per cent of its storage capability for the project's duration, the report says.

Mr Brown also identified the need for a six-metre high, 150m long saddle dam north east of the existing wall to stop water spilling around the main structure.

The report, submitted by consultants Slattery and Johnson, took "three weeks from concept to completion", said Bill Johnson. He said it was appalling such fundamental information was not being made public.

"In relation to water the government is not working for the people. They can't rebuild trust until they start being more open about their intentions and the information they already have to hand," he said.

"The government could have done this (compiled an engineer's report) but they didn't do it. And that's because they don't want people to know."

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