A freshwater scientist says it will take up four days of fine weather to reduce the risk to swimmers from sewage runoff after heavy rains last week. 

More than 20 beaches were closed around the Auckland region due to contamination over the weekend. 

Meanwhile, in Wellington there were 17 beaches that had an "unsuitable for swimming" warning, according to the Land, Air, Water Aotearoa website. 

This included popular beaches such as Scorching Bay, Lyall Bay and Seatoun Beach. 

The Cawthorn Institute's Roger Young said the weather, combined with neglected infrastructure, was leading to high levels of bacteria and virus's in beaches and waterways. 

"There's been runoff from those urban areas [so] there's the risk of bacteria and viruses ... getting washed into the waterways and increasing that risk of illness for swimmers. 

"If we get a patch of good weather for three or four days I think things should be back to more normal situations and the risk of swimming at most sites should be reduced. 

Young said swimmers should check with their local council for closures in rain-affected areas.