After hovering around 99 per cent capacity for four weeks, the Murray-Darling Basin's largest water storage - Dartmouth Dam - is expected to start flowing over the spillway tomorrow.
Inflows from last weekend's rain have pushed the dam above capacity.
MDBA River Management senior director Joe Davis said water would begin flowing over the Dartmouth Dam spillway at low rates and contribute minor flows to the Mitta Mitta River downstream.
Water was also still being released through the valves at the base of the dam.
"There has been plenty of local banter about the possibility of the spill, and I know people have been taking bets on when it might happen," Mr Davis said.
"To begin with, water is expected to trickle over the spillway before the flows build up.
"Given it's been such a long time since the last spill, we expect locals and tourists will be keen to visit Dartmouth, particularly over the school holidays."
Since Dartmouth Dam was completed in 1979 it has physically spilled in four years, each in the 1990s, the last time being 26 years ago in 1996.
"Dartmouth Dam has been considered effectively full since early August when we started pre-releasing water from the dam to manage airspace," Mr Davis said.
"The flow of water over the spillway at this stage is not expected to impact on the height of the Mitta Mitta River downstream, with flows expected to remain within the river channel at Tallandoon."
Dartmouth Dam has the largest capacity of any dam in the Murray-Darling Basin (3,856GL).
At 180 metres, the wall at Dartmouth Dam is the highest dam wall in Australia.
Almost 10pc of the inflow to the River Murray system comes from the Dartmouth Dam catchment.
This is the fifth time the dam has spilled since it was completed in 1979.
River operators work in close collaboration with the Bureau of Meteorology to ensure dam management is guided by up-to-date rainfall and inflow forecasts.
Jointly managed by the MDBA and Goulburn-Murray Water, Dartmouth Dam is the most upstream storage in the River Murray System and collects almost 10pc of the system's inflow.
The Mitta Mitta River flows from Dartmouth Dam into the Hume storage, which is currently at almost 97pc capacity.
Like Hume Dam, the primary purpose of Dartmouth Dam is to conserve water during wetter periods as insurance against future drier times.
Its management is governed by the rules of Basin governments under the Murray-Darling Basin Agreement.
Communities who live, work or holiday near rivers or on a floodplain are reminded to be prepared for flooding.
Stay informed: check the BoM for weather warnings www.bom.gov.au/australia/warnings, listen to emergency broadcasters, talk with your neighbours.
Sign up for the Early Warning Network to get WaterNSW alerts of Hume Dam releases, including Hume Dam www.waternsw.com.au/supply/ewn
