Bird poo is believed to be the reason power was cut to Northland, Transpower has said.

The discovery of large deposits of excrement came during a close examination of Transpower's electricity towers between north-west Auckland and Marsden Point.

The three-hour power cut led to businesses closing across the region and forced Marsden Point refinery to shut down, according to safety protocols. The company said yesterday the outage would cost it between $1.5 million and $2.5m.

Transpower today narrowed the outage to a handful of towers close to Kaiwaka, about an hour's drive north of Auckland.

"The tower where the fault occurred is near tributaries that feed the Topuni River, an area where large water birds live," said grid delivery general manager Mark Ryall.

"Unfortunately, our towers provide an ideal space for them to perch which can interfere with our equipment."

Ryall said Transpower had worked as quickly as it could to find the cause of the power cut, which cut electricity to about 92,000 homes and businesses yesterday.

He said the investigation saw helicopters sent to scan lines and crews of workers at substations along its length.

"Analysis by our protection engineers narrowed the likely location of the fault down to 10 towers, near Kaiwaka.

"We sent crews to those towers this morning and their investigation led us to conclude that the most likely cause was birds."

Ryall said bird deterrent devices would be installed immediately in a bid to stop a repeat situation.

"Unfortunately, birds are always a risk to our network and we already have work underway that aims to identify new innovative solutions to tackle this issue."

The cut happened while maintenance staff worked on one of two circuits feeding the North. The maintenance work meant there was no backup when power went down on the other line.