A man has been taken to hospital in a serious condition after he became caught in a fishing net tonight, his cries for help and flashing light alerting a passerby on a North Auckland beach.

The alarm was raised at 6.28pm and police were told the emergency was unfolding in the water off the peninsula between Algies Bay and Goldsworthy Bay, 11 kilometres south-east of Warkworth, police said.

"Someone has been yelling 'help' and a light has been seen flashing."

The Coastguard, police, including the Eagle helicopter, and St John Ambulance responded, with the Coastguard rescuing the man just before 7pm.

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A St John Ambulance spokeswoman said the man was taken by ambulance to North Shore Hospital in a serious condition.

A police spokesman said earlier he understood the man was possibly suffering from hypothermia.

The man was found behind a catamaran but it wasn't immediately known how long he had been in the water, or how he came to be tangled in the net, the spokesman said.