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Transpower has admitted a massive pylon power cable that plunged on to houses in south Auckland - causing burn damage to three homes - could have killed someone.

Transpower is still working to establish the cause of the breakage that saw the 220kV line end up draped over about 15 homes in Manukau's Richmond Park subdivision.

Residents have reported hearing a loud bang and seeing sparks and smoke when the line snapped this morning.

Transpower Kieran Devine tonight said the line was live when it hit the ground but would have only remained so for "fractions of a second".

"We believe that when it hit the ground it was alive for a very short period of time, because there are scorch marks both on the ground and on fences," he said on Radio New Zealand.

"In one case the grass caught fire."

Mr Devine, Transpower's manager of systems operations, admitted the break could have seriously injured or killed someone.

"We will basically do whatever is needed to ensure, to the best of our ability, that this does not happen again."

Transpower has said it will pay for any damage and Mr Devine said staff had established at least three houses were damaged and it had relocated one household as a result of the accident.

H e said it looked like a joint in the centre of the span of the cable may have failed, sending it crashing to the ground.

Staff did not yet know why the joint had broken, but a major investigation was under way.

Mr Devine said lines usually had a detailed inspection every six months, but he did not know when the snapped line had last been monitored.

The break did not cause any loss of electricity.

Transpower hoped to have the line back up and running during the weekend at the latest.