KiwiRail will scrap its electric trains on the main trunk line between Auckland and Wellington and move to an all-diesel fleet.

Photo: 123rf.com

It said the small fleet of 30-year-old electric trains would be phased out over the next two years.

While the main trunk line runs between Auckland and Wellington, the only electrified section is between Hamilton and Palmerston North.

KiwiRail chief executive Peter Reidy said that meant the company was essentially running "a railway within a railway".

The doubling up of service facilities, inventory, training and maintenance required with two separate systems on the line added to the line's inefficiencies and unreliability, he said.

Transport Minister Simon Bridges said he supported KiwiRail's decision to phase out electric trains on the main trunk line.

Mr Bridges said the driving factor was that the line was only electrified from Hamilton to Palmerston North.

He said it was inefficient to change from diesel to electric to diesel to complete the journey.

"Ultimately, the other option would be to electrify the whole thing, but I'm told that's $1.5 billion worth of work, when you include some of the lines onto it. So I think they've reached, on balance, the right decision."

But the Labour Party said the decision was a backward step and the new diesel trains would create five times the greenhouse gas emissions of the existing freight operation.

Green Party co-leader Metiria Turei said National's underfunding of rail was responsible for the short-sighted decision.

"We know we can do better, we have an electrified rail system, at the moment we should be utilising it, and improving it. Not going back to 20th century technology."

Ms Turei said National should commit to completing the electrification of the North Island Main Trunk Line.