The Royal Commission into the River Murray will be a waste of money if it is not given enough time to call witnesses from interstate, Meningie river advocate Rosa Merlino-Hillam says.
Commissioner Bret Walker is due to complete his investigation by February, but last month requested an extension so he could deal with a High Court challenge from the federal government and Murray-Darling Basin Authority, both of which have refused to cooperate.
State Attorney-General Vickie Chapman denied the request.
Ms Merlino-Hillam expressed her frustration with the impasse when she joined a protest outside a commission hearing in Adelaide last Tuesday.
"We and many other basin community people are appalled at the new South Australian government's refusal to extend the Royal Commission so that it can fully do its job," she said.
"Not allowing the Royal Commission to run its full course would ... make the commission's findings inconclusive and still exclude us all from the truth.
"Why are they fighting so hard to keep us in the dark?"
She also expressed concern that a new organisation claiming to promote "a (Murray-Darling) Basin Plan that provides for people, production and planet" was too closely linked to big irrigators.
Speak Up 4 Water's policies include a cessation of water licence aquisitions and consideration of "more sustainable solutions" for the Lower Lakes and Coorong environment, including a possible lock zero above Lake Alexandrina.