Australian telco giant Optus  is investigating another network outage that led to nine failed triple-0 calls on the NSW south coast.

Optus has issued a statement saying the problem stemmed from a mobile phone tower site in Dapto.

It affected calls between 3am and 12:20pm on Sunday, including calls to the triple-0 network.

Australian correspondent Oliver Peterson says this incident will cause significant damage to the relationship Optus has with its customers.

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