A grim prediction, as Wellington Water crews return to the city's wastewater treatment plant to determine what's caused Wednesday's catastrophic failure.

Heavy rain's caused the Moa Point plant to shutdown, sending wastewater into the capital's South Coast.

Crews are trying to ascertain the full impact of the flooding on the equipment and operations.

Wellington Water CEO Pat Dougherty told Wellington Mornings with Nick Mills it'll be months before people can safely go in the water, as sewage floods out to sea.

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