The Environmental Protection Department has rated water quality at 11 beaches 'good', 17 'fair', four 'poor', and one 'very poor' for this weekend.

Compared with the grading released last week, Kiu Tsui Beach and South Bay Beach have been upgraded from 'fair' to 'good', and Butterfly Beach has been upgraded from 'poor' to 'fair'.

Deep Water Bay Beach, Discovery Bay, Kwun Yam Beach and Silverstrand Beach have been changed from 'good' to 'fair'.

Castle Peak Beach has been changed from 'fair' to 'poor', and Big Wave Bay Beach has been changed from 'poor' to 'very poor'.

The Department said the changes are generally within the normal range of fluctuation of the bacteriological water quality of the beaches. The 'very poor' water quality of Big Wave Bay Beach is due to the accumulated effect of heavy rainfall in the last few weeks, which flushed pollutants from the unsewered beach hinterland into the water.

Seven gazetted beaches - Anglers' Beach, Approach Beach, Ting Kau Beach, Casam Beach, Gemini Beaches, Hoi Mei Wan Beach and Lido Beach - are closed to swimmers year round because of poor water quality.

Many beaches are likely to be more polluted than their grades suggest during and after periods of heavy rain. Bathers should avoid swimming at beaches for up to three days after a storm or heavy rainfall.