KiwiRail's decision to scrap its main trunk electric locomotive fleet will see its annual carbon footprint increase by 10 percent.
Its carbon emissions will increase from 30.8 grams/net kilometre tonne (NTK) to 34 grams/NTK when the new procurement process is complete.
That equates to an extra 12,000 tonnes of carbon each year - or an additional 4709 cars on the road.
The state-owned rail company has chosen Chinese-built diesel engines to replace its antiquated EF locomotives which have run between Hamilton and Palmerston North since 1988.
Electrification of the line was initiated by the Muldoon government as part of its 'think big' movement, and it included the purchase of 22 electric locomotives. Sixteen remain operational today.
The company has released its business case for the locomotive replacement, which concludes an all-diesel fleet is easier to operate and maintain, despite being 20 minutes slower.
It cuts out the need to switch engines at Hamilton, and again at Palmerston North for a train heading from Auckland to Wellington, which takes 40 minutes each time.
Green MP Julie Anne Genter says the decision is short-sighted.
"I think it's a huge waste of money, it's a huge step backwards and it will cost a lot more in the long run," she says.
"If Climate Change Minister Paula Bennett is serious about the long-term future of New Zealand, and doing what is right on climate change, she would not accept this decision," says Ms Genter.
She takes issue with KiwiRail's heavy redaction of the business case, which withholds all information comparing the long-term costs of buying new electric trains versus new diesel ones.
It has also withheld the costs of mothballing the overhead electric lines and the cost of completing the electrification of the North Island main trunk line, and line to Tauranga.
KiwiRail has been asked for comment.