Vodafone's fixed-line internet, including broadband and dialup, and mobile internet were affected, along with email services.
Customers were unable to reach locally-hosted websites, and Vodafone's website was down, even for non-Vodafone customers.
Spokesman Paul Brislen told NZPA one of the company's servers that connected to locally-hosted websites was behind the problem and was being rebooted about 11.30am.
The fault was resolved and internet services were back up shortly after midday.
Work was being done on the server by engineers last night and that appeared to have triggered the fault this morning, although its exact cause was not yet known, Mr Brislen said.
The company apologised to its customers for the inconvenience.
