Chorus' external email was also not working.
The fault has also meant Telecom's own corporate website down.
Telecom spokeswoman Lucy Fullarton told the Herald it was difficult to say how many people had been affected because the fault was also impacting customers' ability to get in contact with Telecom.
The fault is affecting call centres, online services, and internal Telecom networks, as well as some corporate clients.
"It's important to point out that people's ability to call and text has not been impacted," Fullarton said.
All other Telecom customers can still use data, and 111 services are not affected.
Telecom is still unable to give an estimated time to resolve the issue but said technicians are working on the problem.
Fullarton said earlier this morning in an emailed news release that today's outage was unrelated to a Telecom systems' upgrade, which took place over the weekend.
See Telecom's Facebook page with discussion of the fault here.
Earlier this year, the email accounts of tens of thousands of Telecom Xtra customers were attacked by cyber hackers resulting in a deluge of spam from pranksters and fraudsters.
@TelecomNZ no doubt! I feel sorry for the techs dealing with the issue. I'm a corporate cust, @Geni_ICT Helpdesk running off one cellphone!
@nzpilotdude85 No ETA on a fix yet, but we'll update here when we have more info. Regarding your internet issues, where are you located? ^SK
Obviously one of @TelecomNZ 's core switches decided to take the red pill and leave the matrix. This is not good... Not good at all...
Seems like all of @TelecomNZ is down... website + tele support.
We have a technical issue (unrelated to our upgrades) affecting call centres, online services, internal networks. Techs working hard...
...to sort this out as quickly as possible. We're sorry for the inconvenience, everyone. #RPT ^TRF
@TelecomNZ services are down - that'll 'spark' some annoyances. #brand #irony
If ya with @TelecomNZ, you may have noticed issues with your domain services or 0800 number...Not going to be a fun day for their techies X/