The outage began at 3am and was reportedly caused by sabotage.
Transpower said police had been advised after crews discovered the fault on the transmission lines was caused by someone shooting glass insulators high on three transmission towers.
Workers this morning found bullet casings near the damaged pylons.
In a statement, police said Northland CIB had been notified of the damage caused to the three transmission towers, and were making further enquiries.
They added that they hoped to provide an update later today.
Transpower spokeswoman Rebecca Wilson said the damage was in a remote area between Maungatapere and Kaikohe, and bad weather and the rugged terrain hampered access by repair crews, who had to inspect each tower by foot.