Fixing the new wastewater plants in Waipukurau and Waipawa is likely to cost ratepayers between $11.9 million and $20.2m, according to a new report.

The wastewater plants that dump treated sewage into the Tukituki River in Hawke's Bay won't work the way they are supposed to and the council that has spent $8.4 million on them has held "confidential discussions" with their designers.

The new plants use floating wetlands and were finished in 2013-14 at a cost of $6.4m. Central Hawke's Bay District Council has spent a further $2m trying to get them to work better since then.

Supplied Central Hawke's Bay Mayor Alex Walker said she was "extremely disappointed" by the final report on the plants

The plants have not been able to meet resource consent conditions for ammonia levels entering the river and have breached E.coli levels several times with exceptionally high readings. In July the council pleaded guilty to exceeding resource consent conditions at the Waipawa plant and commissioned an independent review into the plants.

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The Waipawa wastewater plant, which discharges to the Waipawa and Tukituki Rivers, will not be able to meet resource consent conditions.

The review, released this week, has recommended solutions that would involve either discharging the wastewater to land or constructing an activated sludge treatment process, which could be for both sites separately or a joint process.

Going with the option of discharging to land would cost an estimated $36m. Building the sludge treatment system would cost between $11.9m and $20.2m.

The consultants said there may be savings by modifying the Waipawa waste stabilisation pond (WSP), but they cautioned against this "given CHBDC's and the wider New Zealand experience with the unpredictability of modified WSPs".

There is an irony in the land option, as this was seen as the solution back in 2012, when the Hawke's Bay Regional Council paid $2.3 m for 130hecatres of land and planted forests near the wastewater plants, as a means of assisting the district council with improving its treatment systems.

But the district council of the day decided that option was too expensive and went with the floating wetland designs.

On Tuesday Mayor Alex Walker said she and the council were "extremely disappointed" by the final report on the plants.

But she said they were confident they now had the information they needed on which to base a long-term solution.

"Performance of the plants has been under question by councillors and community for several years and we now have a robust review that confirms that the current infrastructure is very unlikely to meet consent conditions," Walker said.

CHB chief executive Monique Davidson said the council would now consider both options, "work collaboratively with the Hawke's Bay Regional Council, Central Government and the wider community".

"Given the timing of the independent review it is unlikely that a final solution and costs will be ready to be included in the Draft Long Term Plan 2018-2028, but a top priority of that plan will be to ensure appropriate resources are allocate to investigate options and plan for the preferred approach," she said.

Davidson would not comment on discussions with the designers of the failing plants, Waterclean Technologies, or say whether the discussions were ongoing.

The regional council still owns the forest land near the wastewater plants, which was being turned into a mountain bike park, and has said the land could be re-purposed for wastewater disposal if needed in future.

Central Hawke's Bay is home to just 12,717 people, and is ranked 50th in size out of the 67 districts in New Zealand.