Murray–Darling Basin Authority head of river management, Andrew Reynolds, said locals and visitors to Lake Mulwala would already have noticed changes to water levels since the end of April when the draw down started.

The lake will now be lowered by 4.5 metres.

"New information gathered from a survey of the extent of the highly invasive aquatic weed Egeria densa in Lake Mulwala has found that the weed is present at greater depths in the lake than previously thought,” Mr Reynolds said.

“The original plan to lower the lake by 3.5 metres would not have exposed weed in these deeper locations.

"We have reviewed and considered this new information.

"By lowering the lake further we can target a greater portion of the aquatic weed which was impacting on the lakes' usage, making it harder to swim, fish, launch boats and water ski. 

The lake draw down is expected to reach 3.5 metres by the end of the month.

The water level will then continue to reduce in the first few days of June until Lake Mulwala is 4.5 metres lower than normal.

"We expect to hold the lake at this level until mid-July when refilling will start, bringing the lake back to its regular operating levels by early August,” Mr Reynolds said.

"The lowering will expose muddy lake flats. This will make it difficult to access the water's edge and reduce the ability to launch boats from the boat ramps.”

People will still be able to boat and fish in the main river channel that runs through the lake but will need to launch their boats upstream.