The Murray Darling Basin Authority had upgraded its River Murray System annual operating plan to reflect low rainfall and relatively warm conditions, across much of the southern region.
The MDBA believes dry conditions are likely to continue through to winter, next year.
MDBA River Management executive director Andrew Reynolds, said the unusually dry July to September, typically when the system received most of its rainfall and inflows, had caused the revision.
"We will continue to plan for all scenarios, including very wet conditions, but this update reflects the greater possibility that dry or very dry scenarios will eventuate,” Mr Reynolds said.
"We have been transferring water to Lake Victoria during spring to improve its storage in preparation for the demands of summer, however, under dry scenarios we will look to target a volume less than 350 gigalitres, half its capacity, by the end of May 2019.
"Going into winter 2019 with low volumes held in Lake Victoria provides us with the greatest opportunity to capture any winter inflows that enter the system downstream of Hume Dam.”
With Lake Victoria having a relatively large surface area and situated in a hot and windy part of the Basin, this also helped to increase water availability in 2019-2020, by minimising the loss of water through evaporation.
"Under all dry scenarios we continue to run the river high downstream of Yarrawonga, in anticipation of heavy irrigation demand throughout the Murray,” Mr Reynolds said.
"We are conscious, however, that if conditions remain dry, there is the possibility that demand will reduce as water entitlement holders with an allocation will elect to carry over some of their water to the next year."
Mr Reynolds said another key change in the update was the ability to use Murray Irrigation Limited canals to deliver water downstream of Barmah Choke, where the narrowing of the river limits the volume of water that can pass through.
"We are keenly aware of the difficult times many people are facing due to the drought,” Mr Reynolds said.
“This access helps us to reduce the risk of a shortfall in water delivery during times of peak irrigation demand.
"Environmental water holders have so far used less water than they would in an average year, in part because the usual overbank flows in the Barmah-Millewa forest caused by winter rain did not eventuate and as a result, there was no natural event to extend. In this challenging season, having available water sources limited to Hume Dam releases and the Goulburn River has also impacted our ability to deliver water for the environment at the same time as moving water through the system to Lake Victoria.”
There was the possibility of more environmental water being used, but that would depend on system capacity and the priorities that were identified.
Mr Reynolds said the updated operating plan helped river operators to manage water deliveries and provided a transparent signal to water entitlement holders about the likely risks and opportunities in the period ahead.
"We urge everyone who uses water to plan ahead for all scenarios, including the possibility that conditions stay dry and allocations do not improve. None of us knows with certainty how much water we’ll have in storage at the end of the season—that depends on how much it rains," Mr Reynolds said.
The update is prepared by the MDBA with input from the Australian Government and the Basin states of New South Wales, Victoria and South Australia.