A study into the aquatic ecosystems of the river found the catchment was more than four times overallocated, a recent report to the natural resources committee said.
The report author, council environmental scientist Justin Kitto, said there were 25 surface water and groundwater takes within 100m of the river equalling 2.2cumecs of allocated water on "paper".
One major irrigation scheme, operated by the Arrow Irrigation Company, draws 1.389cumecs from the river. The total primary allocation limit for the Arrow catchment was 0.479cumecs, half the seven-day mean annual flow of the river.
"As the existing primary allocation is 2.2cumecs, the catchment is currently more than four times overallocated."
As it was not known how much water users actually took, it was hoped the government-enforced water metering coming in this year would mean a more accurate assessment would be possible within the next few years, Mr Kitto said.
Spawning and juvenile rearing of brown and rainbow trout were the river's main sports fishery values, so providing sufficient habitat throughout spring and continuous flow in December and January was necessary to ensure recruitment in Lake Wakatipu and the Kawarau River.
"A winter minimum flow should be considered to provide sufficient flows for trout spawning, particularly in the advent of any future water storage in the catchment."
The flow recommended to provide habitat for brown trout spawning is 0.288cumecs.
Modelling suggested a low flow of 0.470cumecs would probably provide for juvenile and adult brown trout, he said.
Having a low flow for the river was not recommended.
