Bird poo is likely to blame for the major power cut in Northland

More than 90-thousand homes lost power for three hours on Wednesday - at the same time maintenance was being carried out on the North's backup circuit.

Transpower's Mark Ryall says the tower where the fault occurred is unfortunately an ideal space for birds to perch, and large water birds live nearby.

He says helicopters were sent to investigate the lines and engineers concluded birds was the most likely cause.

Bird deterrents are being installed in the area.