When full, the dam will hold about 78 gigalitres, increasing the ACT's storage capacity by about 30 per cent. The dam will be built about 125m downstream of the existing Cotter Dam.

These and numerous other details are in Actew's draft environmental impact statement lodged this week with the ACT Planning and Land Authority.

Public comments on the statement are invited over the next 20 working days.

Subject to approval, construction is expected to begin in about October next year.

Before then, in a deal with the ACT Government, Actew will help widen the Cotter Road to accommodate about 20 large truck movements a day to the construction site in the Lower Cotter catchment, about 18km west of Canberra.

Trucks too large for the two bridges will travel via Uriarra Crossing to the site.

As well as the main dam, there will be two adjacent earth rockfill saddle dams about 11m and 16m high.

The project is due for completion in about March 2011.

In its environmental impact statement, Actew says most of the land surrounding the Cotter Dam is of low environmental value and is already affected by the existing reservoir, previous forestry plantations and the severe 2003 bushfires.

Of the land to be inundated, 164ha is former pine forest which was destroyed by the fires. The remaining 105ha is native vegetation.

''It is considered that, on the basis of the commitments made, the risk of significant environmental impacts of the Cotter Dam enlargement is low,'' the statement says.

A quarry will be established on the site and weekly blasting will extract material to be put through a rock crusher for use in the construction. Actew's program manager approvals, Chris Webb, said blasting would be heard about 2km away. A 500m exclusion zone would be established to ensure blasting was safe.

The project requires ACT and Federal Government approval. Mr Webb said all approvals were expected by about July next year.

Actew chief executive Mark Sullivan said although it was theoretically possible the development application would be refused, there was a lot of confidence this would not be the case.

Potentially, the show-stopper was the endangered fish the two-spined black fish and the Macquarie perch.

But there were now strong indications measures taken to protect the fish would improve their habitat.

A previously unknown colony of bats has been found in the precinct of the dam but outside its foreshore.

Mr Webb said very little else of significance had been identified.

Preparation of the environmental impact statement had captured the heritage and natural history of the Cotter area from the indigenous and European viewpoints.

This had included the history of the Cotter Dam as Canberra's original water supply.

Mr Webb said although 105ha of native vegetation would be covered by water, Actew had offset this by planting nearly 1000ha in the lower Cotter catchment in the past two years. Another offset would be to remove some of the native vegetation to be inundated to create habitat for native animals.

Mr Sullivan said water from the existing Cotter Dam would continue to be used during the construction of the new dam. The biggest risk, though manageable, was heavy rain during construction.

''I guess, if we are ever going to see rainfall in Canberra, it will be while we build a dam.''

One of the greatest concerns is the anticipated increasing cost of the dam. Mr Sullivan said costs were escalating in a worrying way.

In the past 12 months, the cost of steel had increased by about 120 per cent and concrete by about 60-70 per cent. The estimated cost of the dam 12 months ago was about $150million.

''It will be north of that number,'' Mr Sullivan said.

For a copy of the environmental impact statement, call Actew on 62483563 or visit actew.com.au