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The North Otago and South Canterbury economies could lose up to $42 million a year and 371 jobs if a water allocation plan for the Waitaki River is not changed, according to an economic impact study.
Two-thirds of farmers who irrigate from the Waitaki River would lose a total of about $30 million a year in farm income.
And allocating some of the Waitaki River's water to Ngai Tahu takes a potential $106 million to $109 million a year and an additional 900 jobs away because of lost future irrigation.
Those figures are in an economic impact assessment prepared for an Environmental Canterbury (ECan) panel of three commissioners sitting in Oamaru to hear submissions on a change to the Waitaki catchment water allocation regional plan.
However, the Lower Waitaki River Management Society, which opposes any change, has challenged the figures, saying they lack detail and are a worst case scenario.
The economic impact study, prepared for ECan by Harris Consulting, looks at the potential effect of pre-2006 resource consents being renewed to meet the existing water allocation plan's 150cumecs minimum flow when they have between zero and 130cumecs in their existing consents.
Consultation which led to the plan change resulted in an agreement by irrigators to relinquish 11cumecs to Ngai Tahu, and the study also calculates the effect of that not being available for future irrigation - about 15,000ha.
This was a significant financial gain the region was giving up by allocating the 11cumecs to mahinga kai.
The pre-2006 consents, which include the Lower Waitaki and Ikawai-Morven-Glenavy irrigations schemes, irrigate 65,700ha, about two-thirds of the area irrigated from the lower river.
However, Otago University senior lecturer Viktoria Kahui said the study assumed ''the worst-case scenario'' which tended to overestimate the impacts.
Giving evidence for the Lower Waitaki River Management Society, she was critical of the lack of information on assumptions made in the report, including the costs of various scenarios.
Different scenarios should have been assessed to provide a range of potential losses and effects.
''As it stands, the information provided in the Harris Report does not adequately support the estimated financial losses,'' she said.
The next sitting day for the hearing is June 22.
The proposed Waitaki catchment water allocation regional plan change is aimed at ensuring those who held resource consents before the plan became operative in 2006 continue to have the same reliability of water supply. It also provides greater certainty when those consents are renewed. Most are either irrigation or hydro generation consents. It will do that by lowering the minimum flow in the river before water is restricted to those pre-2006 consent holders from the present plan's 150cumecs to between 102 and 144cumecs, depending on the time of year. An allocation of 11cumecs from the river is in the plan change for Ngai Tahu to use as mahinga kai enhancement, although the specific uses are not defined.