The government warned people not to swim off Stanley Beach after the so-called red tide was spotted there.

The latest sighting takes the number of affected beaches to 15, and local media said the latest outbreak was the biggest since 1998, when 90 percent of the city's farmed fish were wiped out.

Red tides are caused by the rapid multiplication of microscopic algae which discolour the water.

Authorities said the latest outbreak was caused by a rare but non-toxic form of algae, and no fish deaths had so far been reported.

Although the algae are not poisonous they can deprive fish of oxygen, and authorities urged farmers to monitor the situation closely.

On Thursday, state media in China reported that a toxic red tide had struck near Shenzhen, a booming city just across the border from Hong Kong, prompting fears it could spread southwards.

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