A major Optus outage has left thousands of Hunter residents in the dark without phone service or internet.

The significant regional disruption appears to have impacted several suburbs, including Sandgate, Branxton, Allandale, Singleton, Sawyers Gully and Aberglasslyn.

Optus officials believe the outage is caused by an underground fibre break, leaving more than 25,000 devices without connection.

The organisation’s standard 5G plan is the most impacted, as well as NBN and Optus NB-I0T.

But, 5G+ and 4G remain operational, according to the organisation’s website.

The matter is under investigation, with the next update expected to come at 1pm on Monday 18 August.

Telstra and other mobile providers do not seem to be impacted.

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