People could have been swimming in surf mixed with wastewater and not have known, Auckland Council says.

The council has launched its website Safeswim, which gives the public real-time information on the water quality at Auckland beaches.

Mayor Phil Goff said it was important people knew where it is safe to swim, especially after rain when wastewater systems could overflow out to sea.

He said the old ways of assessing water were inadequate.

"The way in which water was tested meant that you're getting information about the state of the water two or three days [later].

"When what you really want to know is what the real-time safety of the water quality is."

Mr Goff said the council would be spending $6 billion to try to improve the water quality at beaches, including constructing new systems to deal with wastewater.