LAWRENCE SMITH/Fairfax NZ

NO GO: The health warning signs went up at Auckland's Mission Bay beach.

Swimmers have been warned off one of Auckland's most famous city beaches because of high levels of bacteria.

Warning signs have been put up at Mission Bay telling swimmers and beach-users of the poor water quality.

The contamination was discovered yesterday by Auckland Council's Safeswim monitoring programme, which tests water quality weekly at 63 Auckland beaches.

"Our advice is for people not to swim at Mission Bay on Thursday and Friday," acting chief operating officer Ian Maxwell said.

"Our testing indicates ... levels of bacteria [that] suggest a potential risk to human health."

The test result has surprised the council because checks of the wastewater and stormwater networks did not indicate a problem, and no overflows had been reported.

"This may be a one-off incident from an animal source, a boat dumping, or a false positive," Maxwell said.

He encouraged beach-users to swim at other local beaches including nearby Kohimarama and St Heliers.

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Warning signs up at Mission Bay