A perfect storm led to the large sinkhole in Auckland's Parnell

A 13-metre deep sinkhole opened up on St George's Bay Road last September, causing millions of litres of raw sewage to spill into the Waitemata Harbour.

A report by engineering consultancy WSP found seven contributing factors - including the poor condition of the pipe, extreme rainfall events and weakened concrete blocks.

Watercare chief executive Dave Chambers says if one of those seven hadn't happened, it likely wouldn't have opened up.

He says the main two were heavy rainfall and the fact there was a five-metre section of a different concrete block - but it was all of the factors coming together.