Thousands of customers have been without internet services for more than 24 hours after a Telstra cable was accidentally cut in Sydney's northern suburbs, with some likely to be waiting until the end of the week for a fix.

At 11am on Wednesday, customers of a number of internet providers across Sydney and some parts of regional NSW found their ADSL and national broadband network (NBN) services had dropped out. Some corporate customers were also affected, losing service in their inner-city offices.

It's suspected a developer on a large site in Chatswood accidentally cut several cables as part of construction work on an apartment building.

Among those affected by the outage is Goddard & Co Solicitors’ Stephen Goddard, who said his office was connected to the NBN and had been without a landline, fax or internet connection since Wednesday morning. Mr Goddard is an Exetel customer, though the outage has also affected Telstra, Aussie Broadband, Dodo and iPrimus connections.

"I've left the office and gone home, but I still can't get to my server and my files," Mr Goddard said.