A power pylon that fell last Thursday, cutting power to most of Northland was due to contractors removing too many nuts from bolts connecting the tower to a base plate, Transpower chief executive Alison Andrew says.

This occurred during routine maintenance by Omexom on the transmission tower and its baseplate near Glorit, north of Auckland, but which was not to procedure, she said.

"Our view is that the specifications in procedures for this type of work were not followed. All the nuts securing the tower to the base plate on three legs have been removed, which caused the tower to lift off the base plate and fall.

"It is unprecedented and inconceivable that so many nuts were removed at once."

The collapse cut power to 100,000 properties and was "completely unacceptable", the government said.

Andrew said while Transpower was grateful no one was hurt, the failure to follow procedure had a significant impact on the people of Northland.

An external party would conduct an independent investigation.

"The ongoing investigation will look at in much more detail what happened and why the correct procedures were not followed. We are committed to learning from this event and implementing any additional controls that may be identified," Andrew said.

Andrew said the investigation needed to be fair and it was "far too early" to talk about discipline.

The contracting crew doing the work on Thursday when the pylon fell was stood down immediately.

Transpower executive general manager grid delivery Mark Ryall said any work that was more than "removing one bolt" needed an engineering review. He said he was "confident" in the reviews so far.

Ryall said an audit done recently was successful and showed no issues.

WorkSafe is also investigating the incident.

Omexom NZ managing director Mornez Green said the company had "more than 25 years with experience working on and building transmission line powers with and for Transpower".

"What happened last week was unprecedented. As part of routine maintenance base plate and the nuts securing the tower to the foundation were removed, cleaned and treated by Omexon staff. During this work, too many nuts were removed. And the tower fell over on its side. The fallen tower caused the power outage. Omexon promptly activated its emergency response to restore the power to what had become a very complex site."

Omexom would be "open and transparent with all parties involved" in the investigations.

He personally apologised to "all people affected by this incident".