Wellington's Moa Point wastewater treatment plant has been shut down and staff evacuated from the site, after an equipment failure flooded multiple floors.

Untreated wastewater is being discharged into the sea and that may continue for some time, Wellington Water chief executive Pat Dougherty said.

"This is a serious situation and we anticipate the plant will be shut down for an extended period," he said.

Dougherty strongly advised the public to stay away from south coast beaches.