Coogee and Malabar are among 11 beaches in the Sydney region to warrant a "poor" rating for water quality, with rainfall events the main factor, an annual survey found.

The state of the beaches report for 2015-16 found water quality had deteriorated in all three Sydney areas - north, central and south - in the past year. Contaminants include faecal material from animal and human sources.

Severe thunderstorms in November and December last year, and the wettest January in 28 years the following month contributed to the drop in quality.

"Rainfall is the major driver of pollution to recreational waters, generating stormwater runoff and triggering discharges from the wastewater treatment and transport systems," the report said.