Environmentalists say reducing the extent of planned cuts to irrigation in the Murray-Darling Basin will not satisfy the terms of the Water Act.
A guide to the new Murray-Darling Basin Plan released last year aimed to recover between 3,000 and 4,000 gigalitres of water for the environment.
It prompted outrage because of the likely effects on farming and jobs and the authority has told a Senate inquiry it is now modelling cuts as low as 2,000 gigalitres.
Environment Victoria's Amelia Young says that will not be enough water to improve and maintain the system's health.
"We're very concerned about that, yes, because we don't believe that it will restore the ecological connectivity of these very, very important sites," she said.
"The Act clearly states that the plan must secure the ongoing and future health of the RAMSAR-listed wetland sites."
She says a balance needs to be struck.
"The irrigators have been calling for a balanced approach but we what we really think needs to happen is that a balance needs to be restored so that we're not continuing to trade off the environment," she said.
"That means giving enough water back to rivers, not continuing business as usual."