When the anaerobic pond was built, Mr Thrush said the treatment system's performance would improve but there would still be overflows of partially treated wastewater to the river during stormwater overflows.
"If we do not build the stormwater balancing pond there is the potential that we will breach our current consent every time we have a heavy rain event for at least the next five years," he said.
Building the two ponds was estimated at $1.8 million, to be paid for by a loan financed annually from the wastewater renewal account budget set in the 2015-25 Long Term Plan.
Councillors Kelly Annand and Andrew Watts were not in council when the decision to install floating wetland treatment was made and expressed concern that the money being spent on upgrades may be futile if a future government banned discharges into rivers altogether.
"Central government is not keen to have any wastewater going into river systems. Is this something that might have to be re-addressed if we need to get this water out of the river?" Mrs Annand asked.
Mr Thrush responded that land disposal was an option when the wastewater upgrades were first discussed, referring to the HBRC's purchase of a 130ha forestry block at Waipukurau to take the wastewater.
"Part of that decision was to have large protection ponds built to hold the wastewater and stormwater at times when it would not be able to put on land, so these storage facilities would work with that."
Mr Watts said that since he had been on council the mayor and staff had said the facility was working.
"We have since had new chemicals applied, filters put in and now two extra ponds - surely when we looked at this thing we had data of what was coming into this town system, consultation and engineers involved, yet still we are putting expensive Band-Aids on. I would not like to add up the money we have now spent on top in little bits. I believe this scheme is not working and I'm worried about going back to tell the public we made the wrong decision and will have to go to disposal to land."
Committee chairman Ian Sharp responded that the council had been keen on the land disposal option but at $12 million and rising it was very expensive.
"Then a report came to us that the land would only take 48 per cent of the sewage from the pond - the rest would have to be stored and dumped into the river at high flows."
Councillor Sally Butler added that the health of the river was very important to the district.
"Repairing the stormwater infiltration is an ongoing, slow process and we need to do something that fixes the river now."
If a land-based option were to become a reality, the wastewater could not be applied in a raw state, so the council had to ensure it was being treated as well as possible, she said.
A suggestion by councillors Watts and Annand to have another council workshop before deciding on approving the extra storage pond was dismissed in favour of the pair having their own workshop to get up to speed on why the floating wetland decision was made.
The decision on building the anaerobic and stormwater transfer ponds together will go back to the next full council meeting on July 28.
