A mere 48 hours after Wellington's Mayor went for a swim prove the sea was safe, the southern coast has again been declared unsuitable for swimming

On Wednesday, the council announced the city's southern beaches could reopen after water testing deemed the area low risk.

It had been off limits for weeks after a catastrophic failure at the Moa Point treatment plant sent raw swage spewing into the sea.

Now, after heavy rain in Wellington overnight people are being advised not to enter the water, as there may be an increase in bacteria, and a high risk of illness.