Wellington's southern coast could still be back to square one in more heavy rain

Wellington Water's warning there's still a high likelihood raw sewage will flow through the short outfall in a storm.

Beaches on the wastewater plagued-coast have re-opened under caution, after the Moa Point treatment plant catastrophically failed last month.

The number of water quality testing sites has decreased from 23 to 14, as Wellington Water moves to a recovery phase.

It's still not known what caused the disaster.