A former Otago Regional Council (ORC) senior manager is backing an Environment Court appeal by the Lindis Catchment Group (LCG), representing irrigators, who are trying to overturn an ORC decision.

Dr Dean Olsen is an environmental scientist who was the manager of the ORC's resource science team in 2016 when an ORC hearing committee of Cr Gretchen Robertson, Clive Geddes and Richard Allibone released its plan change 5A decision, setting the water allocation limit and minimum flow rate for the Lindis River.

Dr Olsen was an expert witness yesterday for the LCG on day five of its appeal against the decision.

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LCG is proposing an alternative allocation and minimum flow rate - with water for irrigation coming from new bores, referred to as the ''galleries scenario'', rather than from 60km of water races dating back to the 19th century.

The ORC and the LCG reached agreement in mediation that the galleries scenario was the best option for the Lindis catchment, ORC lawyer Alistair Logan explaining to Judge Jon Jackson at the outset of the hearing that changes in the ORC position ''may seem surprising, but they are evidence-based''.

In his written evidence, Dr Olsen said the galleries scenario ''would result in better ecological outcomes in the lower Lindis River than the PC5A decision scenario, except under conditions that are experienced infrequently and for short periods of time.

''It is my opinion that the effects of such infrequent and short duration events are more than compensated for by the higher flows that are expected to occur much of the time under the galleries scenario, compared with the PC5A decision scenario.''

Comparing the effect on the river of the two scenarios, Dr Olsen said the ''most material differences'' were the shift of water intakes about 7km downstream under the galleries scenario.

While the water race intakes have an effect on 22km (or 33%) of the river, the galleries scenario would affect 12km (or 14%) of the river, Dr Olsen said.

He also considered the galleries scenario would result in better water quality than the decision scenario.

The appeal is proceeding because Otago Fish & Game Council and other fishing organisations are defending the plan change 5A decision, arguing it would produce better outcomes for trout than the galleries scenario.

For Fish & Game, Cawthron Institute freshwater fisheries scientist John Hayes said the galleries scenario performed ''more poorly'' than the plan change 5A decision along the lower 17km of the river, and he ranked it the worst of four scenarios.

''The lower the minimum flow, and/or the greater the allocation rate, the greater is the risk that life-supporting capacity, ecosystem processes, fish abundance and growth will decline.''

Dr Hayes said the minimum flow proposed by the LCG represented a ''moderately high risk'' to the passage of adult trout and fell ''well short'' in environmental protection.

''The productivity gains offered by higher flows upstream of the State Highway 8 bridge ... risk being largely negated by poor provision for trout'' in the lower part of the river.