Transpower is investigating why five contractors who sparked a full-scale emergency at a tunnel in Auckland were not aware their radio communications had failed.
Twelve fire engines and ambulance and police staff were called to the Vector cable tunnel in Nuffield Street, Newmarket, on Thursday night when controllers working above ground couldn't make contact with the five men working in the tunnel.
When rescuers went in, they found the men working, unaware of the emergency response above.
Transpower said an equipment problem could have caused the false alarm.
Chief executive Patrick Strange said the tunnel was very dry and inert so although controllers suspected there had been a communications failure they had to assume the worst.
Dr Strange said the company has a strict routine that staff working underground must call back regularly.
The tunnnel was completed in 2000 for power supply cables. It consists of approximately 9kms of tunnelling with three primary shafts and several minor shafts.