A Telstra cable cut during civil works in suburban Sydney has caused an internet outage for some NSW customers, with those affected now without a connection for more than 24 hours.

The ABC understands certain customers covered by iPrimus, Dodo and Exetel are the primary users affected, with some Telstra customers also hit by the outage.

A spokesperson for internet company Exetel said the outage had affected customers since 11:00am yesterday. It is understood the cable was cut by contactors conducting civil works for an apartment development.

Although the internet companies affected have not confirmed when services will resume, or how many people have been hit, the ABC understands many customers have been told services will not be restored until tomorrow morning — meaning some will be without internet for more than 48 hours.

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Lawyer and Exetel customer Stephen Goddard, whose offices are on Elizabeth Street in Sydney's CBD, told the ABC he had to take a forced day off as he could not log onto his internal server.

"It's very frustrating," he said. "It's not like I live out in the middle of nowhere — I'm on Elizabeth Street.

"And the most frustrating part is that some other offices on my level [in the same building] have internet. I had to go home, there's nothing I can do.

"They told me it won't be restored 'til tomorrow — that means I'll be out for 48 hours."

Calls to Exetel's customer support line gave an automated message.

"We're are experiencing a network outage in NSW area and it is currently being investigated," it said.

"We're making our best attempt to restore the service as soon as possible."

Internet parent company Vocus, which owns Dodo and iPrimus, also confirmed they were experiencing issues.

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"Please be advised that a number of data services in NSW are experiencing continued impact from yesterday's fibre break," a Vocus spokesperson said.

"Investigations have identified damage to a large number of fibres across four optic fibre cables.

"Third party technicians are in the process of hauling and resplicing cables 1-3. Replacement cable for the fourth cable is being sourced."

The ABC contacted the National Broadband Network, which said this was not an NBN-related outage and that signals were clear.