ACTEW data shows the estimated final cost of the dam wall and abutment alone has increased by $52.7 million since the project was budgeted in 2009. The documents also show that ACTEW's bulk water alliance partners are set to receive an additional fee of $6.4 million for working on the project.

The opposition is demanding the water utility and its two shareholders, Chief Minister Katy Gallagher and Treasurer Andrew Barr, explain the cause of the "soaring costs" that have led to a final estimated price tag of $409 million. The project's original budget was $363 million.

The government said it was too early to provide a complete breakdown of costs, but its understanding was still that most of the increase was "due to record rainfall, floods and a geological fault".

But ACTEW data, supplied in response to questions on notice, shows floods are expected to account for just $5 million of the additional budget.

A breakdown of the estimated direct costs of the dam construction shows the cost of the dam wall and abutment alone rose from $93.7 million in 2009 to a current estimate of $146.4 million. Of that figure, $3.7 million can be attributed to the geological fault at the base of the dam that ACTEW had to repair.

The data also shows that work on the saddle dams and quarry rose by $2 million to $43.8 million, while structures that were set to cost $27.7 million are now likely to come in at $48.7 million.

Opposition infrastructure spokesman Alistair Coe said the bulk of the additional costs appeared to relate to work on the dam wall and

abutment and not to flooding.

"I think it shows that there's much more to this issue than Andrew Barr and Katy Gallagher have let on," Mr Coe said.

"It's incumbent upon them as the custodians of taxpayers' money to demonstrate the full expense and the reasons that the costs have blown out.

"ACTEW should demonstrate, firstly, a complete breakdown of these costs but they should also advise when they let Katy Gallagher and Andrew Barr know about these soaring costs.

"It seems the taxpayer has taken on the lion's share of the risk in this project."

Mr Coe said the documents also showed that the dam's non-owner partners were originally entitled to a fee of $35.6 million for working on the project.

That fee was set to be reduced to $6.4 million.

Mr Barr said ACTEW had kept the government informed of the rising costs and delays and the shareholders were satisfied with the water utility's explanations for the revised budget.

He said "all advice to the government has been that these increased costs to the project have resulted largely from factors external to the project."

"ACTEW has advised that they will be in a better position to provide a more detailed breakdown of the increased cost of the dam once a final analysis is completed," he said.

"At this stage, it is simply too early to provide a detailed breakdown."

Mr Barr said that he had been "advised that the figures quoted so far do not entirely include the impact of the record rainfall and minor flooding that affected work on the project."

An ACTEW spokesman said the water utility was unable to provide "full details" in response to the "very detailed and complex questions" Fairfax Media asked about the increased budget in time for Fairfax's publication deadline.