The blame game is over - Palmerston North City Council has conceded its sewage treatment plant is responsible for major damage to the Manawatu River.

Twelve months ago the city council was reluctant to take responsibility for a significant loss of aquatic life in the river downstream of its plant, saying it was compliant with its consent and couldn't be blamed.

But at a public meeting at the Convention Centre last night, Palmerston North Mayor Jono Naylor said the council, at the time, had been "at a loss" to work out what was going wrong. It wanted to be sure action was necessary.

A report released yesterday was unequivocal - the Totara Rd plant was 90 per cent responsible for damaging the vital mayfly population downstream. The council breached its resource consent.

Horizons Regional Council chief executive Michael McCartney said the next step was to review the consent to see what changes were needed.

"Then the council will have to look at a work programme to make it happen."

The council treats for phosphorous about 120 days a year, at a cost of $3000 a day, or $400,000 a year. Those costs could balloon. Mr Naylor didn't know what the cost would be to ratepayers until the council was sure what needed to be done and then it could be included in next year's annual plan.

In the meantime, any money needed for extra alum dosing would come out of the operational budget.

"If we find we need a significant amount of money to fix the problem, the community will have to be consulted," he said.

City councillor Chris Teo-Sherrell said he was pleased the denial of the problem was over. "Although it's taken a year, it's good it's now out in the open and the important thing now is to get on and fix it."

Massey University water ecologist Mike Joy said the long-awaited report revealed little and its results were known 12 months ago.

"They might have a bit more data but at the end of the day the results are the same," he said.

"It's the treatment plant that is causing it and they were told that before.

"It's been a waste of time and money, ratepayers' money."

The city council was issued an abatement notice by the regional council last year, but it was later withdrawn and the councils agreed to a joint monitoring programme for the summer. The results were shown to the public for the first time last night.

"It was [previously] confusing as what was coming out of the pipe was complying with the consent, so we couldn't work out what was going wrong," said Mr Naylor.

In April last year, Horizons found significant amounts of algal growth in the river below the treatment plant and the council in breach of its discharge consent for causing adverse effects in the waterway. A second check in June showed similar results.

The green algal growth was believed to be the result of too much phosphorus in the river and the treatment plant not working properly.

Large amounts of algae means "good" bugs such as mayflies cannot feed or breed.

A possible scenario, that needed further research, was what Opus principal environmental scientist Keith Hamill described as "the sponge effect". It was possible that during periods when the plant was not treating for phosphorous, it was being absorbed in the river sediment. As water levels dropped, the absorbed phosphorous was being released in "a lag effect".

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