A leaked document reveals the office of a top water bureaucrat declared a "crisis of confidence" in a plan to save the Australia's most important rivers, amid concerns the states will undermine the environment and $13 billion-worth of taxpayer investment.

The leak - which a government spokesman declared was "disappointing" - coincides with an expected Senate vote this week involving a Turnbull government bid to reduce by 70 billion litres the target amount of water that irrigators must return to the environment in the northern half of the Murray Darling Basin.

The Murray Darling Basin Plan stipulates the amount of water that can be drawn from the river system, aiming to take at least 2750 gigalitres a year from irrigated agriculture and return it to rivers, wetlands and flood plains.

The Murray Darling Basin Plan stipulates the amount of water that can be drawn from the parched Murray-Darling river system. Credit:Louie Douvis

It is underpinned by 33 “water resource plans”: policies devised by the states detailing how water is to be divided between users - mostly irrigators - and the environment. The plans are critical to the success of the broader basin plan.