Nitrogen and phosphorous from fertiliser and animal urine and manure cause nuisance weed and algae growth in freshwater, stripping it of oxygen and affecting instream life.
Ms Keedwell said the hearing was timely, because the council was not using the One Plan to its full potential to improve water quality.
"We have allowed an opportunity to make more gains to pass us by."
The One Plan set limits to nitrogen leaching per hectare per year in its famous or infamous Table 14.2. The limits were set using a computer programme called Overseer.
When it became obvious that farmers were not meeting the table, in June 2013, councillors instructed staff to "only implement the plan as long as there was no economic impact".
Ms Keedwell was the only one to vote against the motion. She wanted the legitimacy of councillors overturning a One Plan rule investigated. Other councillors didn't support her and staff followed the councillors' direction rather than implementing the plan as it was written.
"We took a softly, softly option. We could have pushed harder, with very little impact on the farming community," she said.
Instead staff gave farmers a list of mitigation options, and asked them to say which they could afford to do.
In the final and definitive Environment Court hearings over the plan, Judge Craig Thompson said economic impact was inevitable.
"He said it would be expected some farmers would go out of business because the way they were farming was unsustainable," Ms Keedwell said.
Horizons staff members have said the way the plan is being implemented will lead to an average nine per cent reduction in nitrogen leaching.
Ms Keedwell said that reduction could be just a paper calculation, if farmers haven't done the work. Also, she asks, is nine per cent enough?
TYPICAL NITROGEN LEACHING PER HECTARE/PER YEAR
18kg/ha/year - low stocked dairy farm, mainly grass fed cows, heavy soils
40-80kg/ha/yr - irrigated dairy farm on coastal sand country or high rainfall Tararua dairy farm
40-80kg/ha/yr - average across intensive beef farming with feedlots
100kg/ha/yr - irrigated commercial vegetable growing
LIMITS SET IN ONE PLAN TABLE 14.2
30kg/ha/yr on the best land, dropping to 25 over 20 years
2kg/ha/yr on worst land, remaining at 2 over 20 years