Warning: Auckland's beaches could be flooded with faecal bacteria after downpours

Aucklanders are being warned multiple beaches are unsafe to swim at over fears heavy downpours on Tuesday could cause mass faecal bacteria contamination.

The Safeswim website shows many of Auckland's most popular beaches, including Mission Bay, St Heliers and Herne Bay are highly likely to cause illness.

Auckland is being hit by torrential rain and thunderstorms throughout Tuesday, which could overwhelm wastewater networks and can cause waste-water and sewage to overflow into the ocean.

"Water quality models predict levels of faecal indicator bacteria that exceed national guidelines for swimming," Safeswim says on its website.

Faecal indicator bacteria are bacteria from the gut of humans and animals. They are introduced to the environment via animal and bird droppings, or wastewater discharges and stormwater runoff. Their presence in water indicates other harmful pathogens such as E. coli could be present.

Auckland Council advises that swimmers should wait at least 48 hours after rain before swimming in the water.

The high-risk warnings are expected to lift on Wednesday evening. 

The swimming warnings follow a large low front moving across New Zealand, bringing torrential rain with it. 

Heavy rain is also expected in the Bay of Plenty and Taranaki.