The NSW Office of Water says big rain in northern catchments will boost inflows to the Menindee lakes, but the effects are uncertain.

Around 20,000 megalitres are expected to reach the lakes mid-next month, at the same time as warm weather creates favourable conditions for algal blooms.

The Office of Water says it is not clear whether the new flows will disrupt the blooms.

The department expects the influx might help for a short while, but ongoing monitoring will be needed.

But the boost to the Menindee Lakes will not offset the level of water lost from evaporation.

The Office of Water said losses from evaporation in the lakes were close to 20,000 megalitres in January, but the inflows will not fully replace that amount.