The details of a settlement between Masterton District Council and a consulting firm which underestimated the cost of the Homebush sewerage upgrades are being kept secret.
The council announced on Tuesday a settlement had been reached with Beca Carter Hollings and Ferner (Beca).
It was also revealed that the $30 million wastewater upgrade project would be delayed five months after bad weather and a change of materials.
HDPE plastic liner will now be used to line the ponds instead of silty clay.
Councillors were told in January 2011 that Beca had underestimated the cost of the project by up to $6.6 million. The council has since been in negotiations with the company, and has switched consultants.
The council also announced on Tuesday that it has reached an agreement with Hopper Construction, the company carrying out the work. Hopper referred all media enquiries to the council, which wouldn't give any details of the agreement for reasons of commercial sensitivity.
Masterton Mayor Garry Daniell said in a statement that although sample data was collected at the outset, it was impossible to know for certain if the silty clay liner option was viable until they started digging. The council has now opted for the same type of HDPE plastic liner recently installed at Riversdale.
Mr Daniell said the changes had been made in the best interests of the project but it would delay it by five months.
"This delay is unfortunate but necessary. MDC has always committed to the environmental benefits of the upgrade and has never backed down from achieving them," Mr Daniell said. "The most important thing is that this project is completed and completed properly for the benefit of current and future ratepayers."
The project has contributed a 5.8 per cent rates rise this financial year. A council spokesman said yesterday the $30 million budget figure was still current, but did not include risk contingencies. The council anticipates the project will still come within the current rated provision.
Chief executive Wes ten Hove said the council would provide a full account of project costs upon completion, expected to be about May 2013.
"We are still working through which cost details can be released at this stage, so as not to breach the terms of the agreement or compromise MDC's commercial negotiating position for the balance of this project."
The council said it was protecting the ratepayers' financial interests by preserving its commercial position on this project until it is completed.