An independent review of the Central Hawke's Bay wastewater systems is under way the CHB District Council was told yesterday.

This was while discussing a special agenda item on the charges laid by the Hawke's Bay Regional Council the day before.

The CHB council is facing three charges laid by the regional council in relation to discharges of contaminants from its Waipukurau and Waipawa wastewater treatment plants.

Chief executive John Freeman told councillors it was expected the plants would go over the annual allowable five exceedances, which he believed happened during downpours in September this year.

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"We expected this in the next rain event and pre-empted it by approving building a water balancing pond at Waipukurau," he said.

"We have been working with the Hawke's Bay Regional Council on trying to solve stormwater inundation problems."

The exceedances are tallied on a 12-month rolling programme, and he said if there were no more through to the end of December of January the number would drop back to two at each.

Stormwater was causing problems at Waipukurau, he said, but not at Waipawa where the pond was twice the capacity needed. And as of Monday the plant had not been operating at all because there was not enough effluent going through it, he said.

Councillor David Tennent called for an independent review of the treatment plants.

"In light of the charges it's important that the CHB council takes this seriously and is pro-active," he said.

Mr Freeman said an independent report by Niwa was under way, in agreement with the regional council.

This started two weeks ago, and council staff were asked to come back to councillors with a report on this review at a workshop in two weeks.

Meanwhile, Mr Freeman said the council had taken legal advice on the regional council's prosecution and expected to get the papers on the case either today or Monday.

"Then we will better understand what we are being charged for, why and how."

At this point, the regional council has said that one charge relates to an E. coli discharge at Waipawa, and the other two to an E. Coli and dissolved reactive phosphorus discharge at Waipukurau, both in excess of the five annual allowable exceedances.

The floating wetland treatment plants were commissioned in June 2012 to meet new discharge standards by 2014, but there had been ongoing issues with stormwater infiltration at Waipukurau, and correct chemical dosing at Waipawa.