The Environmental Protection Department has rated water quality at 21 beaches 'good' for this weekend, with 12 more 'fair'.

Compared with last week, water quality at Big Wave Bay Beach was upgraded from fair to good. Discovery Bay was downgraded from good to fair.

Seven gazetted beaches - Anglers', Approach, Ting Kau, Casam, Gemini, Hoi Mei Wan and Lido - 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 heavy rain. Bathers should avoid swimming at beaches for up to three days after a storm or heavy rainfall.