The company considered step 2 was inserted into the zone implementation programme late with no ecological justification and no assessment regarding the reliability of water or the economic implications for users.
Crossman said many farms would not be able to withstand adverse market, climate or regulatory changes.
''The flow-on effect will be a reduced on-farm investment and inability to invest in more efficient sustainable farming methods or infrastructure.
''This will also flow on to a significant regional-wide impact from reduced farm spending and employment as a consequence of lower farm production and income.''
The Fairlie Basin area would be particularly hit economically because of the percentage reductions in nitrogen in plan change 7.
''Opuha Water and Dairy NZ are working together to assess what this 'double whammy' effect is.''
ECan has said the rules proposed in plan change 7 will ensure the Canterbury land and water regional plan ''responds appropriately to new directives from central government, emerging environmental issues and changes in matters that are strategic priorities for ECan''.
Some rules applied over all of Canterbury, while others targeted only the Orari Temuka Opihi Pareora (Otop) and Waimakariri zones.
As far as the alleged ''oversimplified'' approach by ECan in plan change 7, Crossman said it specified that only a ''flow restriction'' regime could be entered into at the start of each calendar month.
''This fails to recognise that climatic conditions and water demand can change significantly over a month.
''The preferred option would be that a restriction regime can be entered into on any day of the year, assuming the appropriate lake level, snow storage and inflow thresholds have been crossed.''
Other concerns of Opuha Water are:
• Operational impracticalities failing to recognise operational constraints of the scheme infrastructure and irrigation infrastructure of shareholders.
• Ineffective use of stored water. Insertion of 2030 increases in minimum flows would draw down the lake faster, reducing overall volume available to achieve minimum flows and provide water for irrigation.
• Worse outcomes for the health of the Opihi River system.
• Opuha Water has about 260 mostly farmer/grower shareholders irrigating around 16,000ha.
Opuha Dam, 17km northeast of Fairlie, has a 50m-high earth dam, with a single hydro turbine and a lake covering up to 710ha and storing over 74 million cubic metres of water.
The proposed plan changes are out for public submission until September 13 after which a public hearing will be heard before an independent panel, probably in the first half of next year.
