Auckland Council is being told to do more to warn people about dirty swimming water

It follows a rare black rating for the popular spot of Okahu Bay - meaning a 'very high risk of illness' for anyone taking a dip.

Direct contamination with human wastewater is the cause, but while the rating was available online a number of people went swimming.

Orakei Local Board chairman Kit Parkinson says the Bay urgently needs an electronic sign, which can quickly be updated.

He says the beach is used frequently by kayakers and kids learning to sail.