New Zealand's reliance on Southern Cross' fibre-optic network has been highlighted by another cable break.

Southern Cross Cable has been forced to divert internet traffic on its cable network connecting New Zealand, Australia and the United States for the third time this year.

Spokeswoman Rosemay Foot said its cable network was cut in Niles Canyon, near San Francisco, on Tuesday, New Zealand time. She was unaware of the cause but said the cable had since been fixed.

Southern Cross redirected communications traffic that would normally flow direct between the United States and Australia through New Zealand during the outage. New Zealand customers were not affected, she said.

The incident follows two other cable-breaks on US segments of its network in February and May.

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Although all the breaks were on-shore, Telecommunications Users Association chief executive Craig Young said the latest incident "reiterated its concern" about the country's reliance on a single trans-Pacific cable network.

If the network had been cut separately in another place before the first break had been repaired, New Zealand internet traffic might have needed to be redirected via Asia, impacting performance, he said.

Foot said Southern Cross' mitigation measures worked "exactly as they are supposed to."