Suze Keith told the hearing climate change had not been sufficiently taken into account by the Lindis Catchment Group (LGC) in its proposal for future irrigation.
Ms Keith is a climate change adviser for the Wellington Regional Council, but appeared in a personal capacity at the hearing.
Ms Keith told Judge Jon Jackson and two commissioners the proposal, to replace existing water races with bores and maintain a minimum flow of 550 litres per second in the river, would not meet the group's need for reliability of supply, leaving the irrigation system in a "fragile position''.
She referred to a 2017 report done for the Central Otago District Council which predicted by the end of the century "a considerable reduction in mountain snowpacks and resultant water storage'', with precipitation falling as rain rather than snow.
It also predicted 10-64 more summer days each year with temperatures above 25degC - about double the current number.
Ms Keith said the lack of "drip feed of snow melt'' over the summer irrigation season would mean a lack of reliable water for pivot irrigators and heighten the "vulnerability'' of the river.
While not "here to do away with farming'', she told the hearing farmers should consider land uses that were "appropriate to the amount of water available''.
Ms Keith was called as a witness by lawyer Maree Baker-Galloway, acting for the Otago Fish and Game Council which is seeking to uphold the Otago Regional Council's plan change 5A decision requiring a minimum flow of 900 litres per second in the river.
The council is appealing its 5A decision, alongside irrigators represented by the LCG.
In answer to LCG lawyer Phil Page, Lindis Valley farmer Tim Davis told the hearing he was not aware there were major snow packs in the Lindis catchment that "stayed around''.
And, as snow melt occurred in spring, rather than during the summer irrigation season, he did not see the relevance of Ms Keith's argument.
It was his understanding climate change was likely to bring warmer, wetter weather with more rain in summer.
The hearing is expected to conclude this week.
