Parts of the South Island have gone offline after fibre cables were accidentally cut.

Hundreds of people have reported internet or phone outages across Southland and Otago.

Website Down Detector found spikes in reports for internet outages across One New Zealand, Spark, 2degrees and Slingshot shortly after 10am.

One New Zealand said engineers were working to restore internet and mobile service in Dunedin.

"This is due to two isolated incidents where fibre cables into the area have been accidentally cut by third parties," the company said.

"Eligible mobile customers will still be able to text using our satellite TXT service."

One New Zealand hoped to have internet restored soon.

The Contact Energy website said it was aware of the outages and its technical teams were working to resolve this as a priority.

In the meantime, the company said people should not unplug, power off or reset their modem.