Transpower says a visual inspection of its infrastructure showed bird excrement as the most likely cause for the three hour outage that left nearly 90,000 Northland properties without power.
A high-voltage circuit breaker tripped yesterday morning, affecting water supply, sewer systems and traffic lights. The Marsden Point Oil Refinery also had to completely power down for safety reasons.
Grid Delivery General Manager Mark Ryall said birds were the likely culprit for the outage.
"The tower where the fault occurred is near tributaries that feed the Topuni River, an area where large water birds live. Unfortunately, our towers provide an ideal space for them to perch which can interfere with our equipment."
Mr Ryall said an investigation commenced immediately following the outage to identify the cause.
"Our priority is always to ensure all people are safe and restore power to communities.
"Once the power supply was returned to Northpower and Top Energy, we dispatched a helicopter to check the lines between Huapai and Marsden while crews were redirected to the affected substations.
"Analysis by our protection engineers narrowed the likely location of the fault down to ten 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.
"We will be installing bird deterrents in the area immediately. 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.
"We apologise once again for this disruption to households and businesses in the region."