Authorities are yet to establish the cause of a fish kill in the Swan River.

More than 70 fish were found floating in the river near the Barrack Street Jetty yesterday.

Lake Monger is being examined as a potential source, as well as waste water from Perth work sites.

Both discharge water into a drainage system which leads to the river outlet near Barrack Street Jetty where the fish were discovered yesterday.

A preliminary analysis is due to be completed this afternoon, but investigators will be on the lookout for more dead fish as today's river tides drop.