The NSW government is investigating the cause and extent of "shocking" algal bloom events in the state's far west that may have killed a million or more fish.

The Department of Primary Industries has sent staff to examine the Menindee Weir Pool region, south-east of Broken Hill, where banks are lined with dead bony bream, Murray cod and other species.

It's the largest of four recent fish kills in NSW after one in the same region at Christmas and two others on the Namoi and Lachlan rivers amid on-going drought and record heat.

"Unfortunately, this won't be the last," Niall Blair, NSW's fisheries minister, said. "There's just nothing coming into the system."