A special tribunal recommended in 2010 the existing water conservation order on the river be amended to ban damming, to protect the Gollum galaxiid - a species found only in the Nevis.
That recommendation is now the subject of an Environment Court hearing in Queenstown.
Pene Williams, counsel for the Director-general of Conservation, Al Morrison, told the court this week Mr Morrison remained "neutral" on the special tribunal report.
However, evidence would be presented by the Department of Conservation, as part of its role was to advocate for the conservation of freshwater fisheries generally.
The Nevis River catchment was home to a native non-migratory galaxiid fish, which was variously known as the Gollum galaxiid, the Smeagol galaxiid and the Nevis galaxiid, Ms Williams said.
Last year, the Director-general commissioned a study by Shannan Crow into the shape characteristics of the Nevis galaxiid compared with the Gollum galaxiid. That research revealed there was a "significant difference" between the two fish, she said.
Dr Crow will give evidence later in the hearing, as will Pete Ravenscroft, who is carrying out research on the Nevis galaxiid.
That work programme on the Nevis galaxiid is to gain more information on its population, its spawning habitat and other characteristics of its behaviour "to ensure appropriate management actions are taken to protect the fishery", Ms Williams said.
Fish experts agreed the Nevis galaxiid was separate and genetically distinct from the Gollum galaxiid.
If a dam was constructed and there was no impoundment beyond the Nevis crossing, the experts had agreed the greatest threat to the ongoing survival of the Nevis galaxiid was predation by trout, she said.
That information had been gleaned since the special tribunal considered the matter.
The galaxiid known by the tribunal as Gollum was the Nevis galaxiid.
