In figures disputed by the ACT government, the Independent Competition and Regulatory Commission says that the Integrated Waterways Project eventually had $50.8 million of territory taxpayers' money assigned to it after original estimates of $3.5 million five years ago.

But according to the office of Environment Minister Simon Corbell, the commission has got it wrong, that $37 million was spent by the territory on the water infrastructure and projects in the inner north and that the commission has included in its figures $11 million that was never spent.

The inner-north pilot network in the Sullivans Creek catchment links a number of man-made stormwater detention ponds - Flemington Road Ponds and Dickson and Lyneham Ponds - and a circulation network intended to provide water to irrigate recreational facilities.

But in its 2011-12 annual report, the ICRC questions the value for money achieved by the project, begun in 2007 and originally conceived to re-use stormwater on playing fields. ''This initiative was initially budgeted to cost $17 million, $3.5 million of which was to be met by the ACT government,'' the report said.

''As at June 2012 the program is budgeted to cost $61 million, $50.8 million of which is to be met by the ACT government.''

The commission also found that the initiative, which was part-funded by the Commonwealth, made little financial sense when the territory's dams were close to full, and recommended that similar, smaller scale projects in the city's south be shelved.

The report represents the third time this year the ICRC has put its criticisms of the cost of the network in writing, but a spokesman for Mr Corbell's office said that the Environment and Sustainable Directorate only became aware of the commission's position when approached by The Canberra Times. ''The ICRC has reported this figure incorrectly,'' the spokesman said. ''The ACT's agreed contribution to the project was $6.8 million to create the necessary infrastructure for stormwater harvesting and reticulation, including for piping, tanks, systems and metering etc.

''The ACT government has slightly exceed this figure, which has increased to $7.5 million for additional measures aimed at improving water quality through stormwater harvesting.''

The ICRC report referred to the amount that had been budgeted, not spent and it failed to take into account money saved by the scrapping of the Weston and Tuggeranong networks.

''The ACT's contribution of $7.5 million was its contribution to the creation of infrastructure for stormwater harvesting,'' the spokesman said.

''The Commonwealth has contributed $10.2 million.

''The $50 million referred to in the ICRC report is for specific wetlands projects itemised below, which has been revised down to an actual figure of $38.87 million.''