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Beaches with highly developed hinterlands are likely to be more polluted than the grades suggested during and after heavy rain.
Principal Environmental Protection Officer Dr Malcolm Broom said: "Bathers should avoid swimming at such beaches for two to three days after a storm, or even longer if the weather remains overcast, shorter if there is strong sunshine."
However, the bi-weekly grades give a general picture of the most recent water quality at bathing beaches at the time of reporting and form the best available forecast for the immediate future.
The system for grading beach water quality is as follows:
Grade "1" indicates that the water quality is good. The E. coli count is no more than 24 per 100 millilitres at each beach so graded, and the expected risk of minor illness to swimmers is undetectable.
Grade "2" indicates that the water quality is fair. The E. coli count is no more than 180 per 100 millilitres at each beach so graded, and the expected health risk is no more than 10 cases of minor illness per 1,000 swimmers.
Grade "3" indicates that the water quality is poor. The E. coli count is no more than 610 per 100 millilitres at each beach so graded, and the expected health risk is no more than 15 cases of minor illness per 1,000 swimmers.
Grade "4" indicates that the water quality is very poor. The E. coli count is more than 610 per 100 millilitres at each beach so graded, and the expected health risk is more than 15 cases of minor illness per 1,000 swimmers.
The decision whether or not to close a beach to swimmers is based on a judgment of what degree of pollution is acceptable.
Normally, the closure of a beach would only be considered by the Urban or Regional Council if a grade "4" occurred repeatedly, so that the average health risk over the bathing scason exceeded 15 cases per 1,000 swimmers.
At present four gazetted beaches, namely Anglers', Castle Peak, Old Cafeteria, and Rocky Bay, are closed to swimmers. The decision to close the beaches has been made by the Regional and Urban Councils on the basis of beach water quality monitoring data for 1994. The public are advised not to swim at these beaches. They are identified by an "X" in the following list.