FLOODS in southeast Australia have flushed a slug of salt and "black water" down the Murray River.

The river's salt levels hit 278 electrical conductivity units at Swan Hill last week, compared with 50-60 EC units in June.

Salinity levels last hit 278 EC units at Swan Hill in 2002.

Murray Darling Basin Authority chief executive Rob Freeman previously warned the floods would dissolve and carry large quantities of salt off the basin's floodplains back into rivers.

Much of the salt has entered the Murray from its Victorian tributaries, the Loddon and Campaspe Rivers.

Last week the Loddon River's salinity reached 381 EC units and the Campaspe 471 EC units.

Further upstream, the Murray River salt load drops to 40-70 EC units.

At Yarrawonga, the salinity levels are just 68 EC units.

The MDBA previously warned the floods would continue to push wedges of black or oxygen-depleted water back into the Murray.

Black water occurs when water flows out on to floodplains stimulating the rapid bacterial breakdown of leaf litter, which in turn depletes the water of oxygen.

The MDBA said while blackwater was a natural event, its impact was more severe this season due to the prolonged drought and build-up of leaf litter in forested floodplains.

Last month it said blackwater was expected to affect the Murray, Edward, Wakool river system, Loddon, Goulburn Broken and Murrumbidgee rivers and the Lower Darling Anabranch "and may worsen in some locations as water temperatures become warmer".

Normally the impact of the blackwater event could be offset by releasing dilution flows down the basin's rivers to boost oxygen levels.

However, the MDBA said most rivers were operating at full channel capacity "preventing further dilution flows as a management option".