Eleven dairy cows were electrocuted when lines on an ageing power pole broke and fell on to live 400 volt lines sparking concerns unmaintained lines across Northland could kill again.

The cows were grazing on a Dargaville property when the line fell and they were killed about 8pm on Wednesday.

A representative for electricity distributors has warned ageing, rotting and rusting power poles on private urban and rural properties were dangerous and the risk of people being electrocuted or hit by falling poles was growing every year.

Northpower line mechanics were dispatched to the Dargaville farm after the farmer contacted the power supplier and they were back at the property today.

Northpower spokesman Steve Macmillan said it was not clear how the stock were electrocuted as Northpower was not on site at the time but clearly they had come into contact with the downed private service line.

Macmillan said in March this year Northpower disconnected a service line to an old cowshed at the property due to safety concerns.

Early indications are that the line that came down was one of the disconnected service lines as a result of an insulator breaking off a cross-arm on a power pole on the line.

He said that line then sagged on to a nearby live 400 volt line, another service line on the property, which livened the previously disconnected service line.

"The service line was left in place after disconnection because the service line is the asset of the farmer. It is the land owner's decision to remove or upgrade their own lines and poles."

Mcmillan said compensation from Northpower would not be an option for the farmer as it was a private service line which was not part of Northpower's electricity network.

"Private service lines are the land owner's responsibility to maintain. It is really important that people maintain the safety of their service lines which is generally the power line that runs from the roadside of the property to the dwellings or sheds."

The call for landowners to maintain poles and lines was echoed by a spokesman from a group representing 27 New Zealand electricity distributors.

"Someone is going to be electrocuted, someone is going to get killed," Electricity Networks Association chief executive Graeme Peters said.

"We're particularly concerned about children."

The Electricity Networks Association (ENA), the voice of 27 New Zealand electricity distributors such as Northland's Northpower and Top Energy, said that in cities and on farms much of the infrastructure carrying electricity from the road power connection to houses, buildings and dairy sheds was getting old and in "a bad state".

The infrastructure causing alarm included aged power poles, crossbars, insulators and rusting metal fittings, which was highlighted in the latest Northland incident.

"These assets are dangerous," said Peters.

Peters was not surprised by the incident in Dargaville and said over the coming years as lines and poles continued to deteriorate it was likely more animals or people would be injured or killed.

"We're going to see a lot more line failings and that raises the risk of people getting hurt, by being electrocuted or by poles falling on them."

The problem has become acute because of lack of maintenance and repair by property owners who don't realise that by law, and as interpreted by the Commerce Commission, it's their responsibility to maintain, repair and keep trees away from poles and lines on their land, he said.

John Blackwell, Northland Federated Farmers President, said there was a lack of understanding about who owned lines and poles but that wasn't limited to the rural community but also those living in the heart of Auckland.

"A lot of people don't understand where their responsibility for maintenance starts and ends."

He urged farmers to check poles and lines on their property regularly.