The NSW government prevented any review from changing water access rules for businesses along the Darling River even if fish populations were being harmed, a confidential document has revealed.

According to a 2012 ministerial briefing document seen by the Herald, a restriction was placed on any review of the rules during the first five years of a new 10-year water-sharing plan so as to provide "certainty" to businesses.

Carcasses after a mass fish kill in the Darling River at Menindee. Credit:Nick Moir

The document noted changes made after five years would have to factor in "socio-economic impacts" and have "no substantial impact" on long-term extractions under various licence classes.

A spokesperson for the Department of Industry Water denied it had tried to block a review from making wholesale changes to protect fish species.