"We have also implemented some hugely successful in-lake initiatives like alum dosing to help give us some shorter terms gains in water quality," Mr Murray said.
"We have seen significant improvements in water quality in the lake and we have, for now at least, hit the water quality target that was set with the community. However, we cannot kid ourselves that we have solved the problem for the long term.
"It is important that the Rotorua community get the facts about what is happening with the lake and what is being done to provide for long term improvement in water quality.
"While we are pleased with how water quality in Lake Rotorua has improved, we cannot say the job is done.
"The science suggests that this improvement has primarily been achieved through in-lake interventions like alum dosing and have been helped along by climatic conditions."
However, the science also suggested that interventions like alum dosing were not sustainable long-term strategies.
"It is not affordable or palatable to continue doing these interventions indefinitely. They are costly and don't resolve the issue at the source. The bottom line is that if we stop our in-lake interventions such as alum dosing lake water quality will decline again," Mr Murray said.
"Robust science and modelling has told us that we must get the nitrogen load down from the present 755 tonnes per year to 435 tonnes.
"It also tells us that the majority of this will need to come from reductions in nitrogen losses from farms in the catchment."
The management of the Rotorua Te Arawa Lakes Programme is supported with science and modelling carried out by the University of Waikato. Lakes Restoration chairman Professor David Hamilton leads research to provide options to preserve or restore lake water quality.
He said that reductions in nutrient loads going into Lake Rotorua were required to maintain stable water quality and minimise the risk of algal blooms and de-oxygenation of bottom waters.
"Our lake modelling and Niwa's catchment modelling have converged on a sustainable catchment nitrogen limit of 435 tonnes per year," Professor Hamilton said.
"While current in-lake interventions have a role to play while catchment actions begin to take effect, they involve considerable cost and carry some risk.
"It remains essential to reduce nutrient loads to prescribed levels that will support the long-term sustainability of the lake."
Mr Murray said: "There is no doubt that the needed reduction will come at a cost to farmers, but at the moment there are no rules to ensure that this cost is shared equitably and that everyone does their bit."
Additional rules were needed to ensure the sustainable nitrogen limit was reached for long-term lake water quality.
"We acknowledge and applaud those farmers who have changed the way they use or manage their land, and have made nutrient reductions on their properties," he said.
"While all these changes will have helped, we know that changes made on individual farms to date are unlikely to be anywhere near the magnitude required to reach the sustainable nitrogen limit and achieve a sustainable long-term improvement in water quality in the lake across the entire catchment.
"Substantial changes will need to be made to reduce farm sources of nitrogen to meet the 435 tonnes nitrogen target."
It would take time to make the changes needed.
"The nutrient reductions needed and the timeframe to reach them were agreed by Federated Farmers Rotorua, Lake Rotorua Primary Producers Collective and Bay of Plenty Regional Council when the Oturoa Agreement was signed in February," Mr Murray said.
"This committed us all to work together to reduce nutrients to the lake to meet the sustainable limit by 2032.
"This agreement set out the principles and processes that will be used to reach the nutrient reduction target. It also agreed to on-going reviews of the science to ensure our work is adaptive to any changes in knowledge.
"We don't want landowners to make changes that are not needed for water quality. But the current science and modelling recommends that the nutrient limit needed is 435 tonnes, and that is what we are developing rules for.
"Rules to reduce nitrogen losses are coming for landowners in the Lake Rotorua catchment.
"We are conscious of the economic impact our decisions may have on the agricultural sector, and are working to ensure these are minimised.
"We are working with the Lake Rotorua Stakeholder Advisory Group with representatives from the pastoral, water quality, forestry and iwi sectors.
"This forum has made significant progress to agree on an approach for rules and incentives for the Lake Rotorua catchment."
To keep informed on progress, go to www.rotorualakes.co.nz and sign up for updates.
