Council infrastructure services executive manager Julie Muir said at the council's three waters infrastructure committee meeting on Wednesday it had been a "bit of a failure" that staff had not communicated the jump in spending from $180,000 to almost $1million for the desludging of the Omakau wastewater ponds.
A council report said costs for desludging the plant's first pond were known in December last year and should have been relayed to councillors then. The costs for the second pond were known in May.
However, the $800,000 overspend had been needed for a "situation that arose that needed urgent attention", Ms Muir said.
All the sludge had to be removed when it was discovered the liner of the ponds was inadequate and needed replacing, and flooding in 2017 and 2018 contaminated the ponds with grit, silt and gravels, meaning all the material had to go to landfill.
Deputy mayor Neil Gillespie said the issue was "a little bit redundant", as it was clear the additional spending had been needed.
However, he recommended the issue be referred to the council's audit and risk committee, for discussion about ensuring council systems would communicate overspends to councillors sooner.
The Omakau wastewater treatment plant, which comprises two oxidation ponds, is on the edge of the Manuherikia River, between the Omakau township and Ophir Bridge Rd.
The ponds were constructed in 1965 and very little maintenance or improvement works had been undertaken since then, a report from water services manager Quentin Adams said.
Desludging of the ponds typically occurred every 20 years, but it was believed the Omakau ponds had not been desludged since they were established, Mr Adams said.
