The Murray-Darling Basin Authority shifted its focus towards irrigation when Barnaby Joyce became Minister for Agriculture and Water Resources, an inquiry is to be told.
Mal Peters, former chair of the Northern Basin Advisory Committee, will appear before South Australia's Royal Commission into the river system on Thursday.
The committee was tasked with developing the Northern Basin Review, which was handed over to the Murray-Darling Basin Authority (MDBA) in October, 2016.
In a written submission, Mr Peters said when he started in the role the MDBA aimed to find a balance between competing environmental, social and economic outcomes.
But Mr Peters said the focus shifted when Barnaby Joyce was appointed as Minister in 2013.
"At that time, it appeared to me that the MBDA shifted its approach further towards irrigation interests," he said.
"Post Joyce, the MDBA Board appeared extremely reticent to make tough decisions."
The royal commission was established in January 2018 to inquire into the operations and effectiveness of the basin plan after widespread reports of water theft.
Its final report is due by February next year.
