UPDATE 9.45am: Telecom says the problem has been resolved.
Some affected customers might have to turn their modem off and on again to resume service.
The outage was caused by problems with planned server maintenance.
Telecom says approximately half of its broadband customers are having trouble connecting to the internet this morning.
The problem arose early today during planned maintenance on one of Telecom’s broadband servers, the company says.
The problem first hit around 6.30am.
A "portion of affected customers" are back online, Telecom says.
The company is working to restore other connections but has supplied no estimated time.
"The issue was quickly identified and our technicians have been working over the past three hours to resume connectivity.
Telecom is issuing updates via its Twitter account.
A spokeswoman told NBR the problem appeared to be nationwide.