Official papers show Transpower is stockpiling a harmful greenhouse gas to avoid paying charges it would face from 2013, under the Emissions Trading Scheme.
The documents show Transpower is importing large quantities of sulphur hexafluoride (SF6) which is used in electrical equipment and is worse for the atmosphere than carbon dioxide.
In 2011, the national grid operator brought in almost 4000 kilograms of the gas - a five fold increase on the previous year - and it expects to import 3000 - 4000 kilograms this year to meet the needs of its work programme
Transpower estimates early bulk purchasing will save it $2.8 million, because later it would have to pay an ETS levy for sulphur hexafluoride of $598 per kilogram, assuming an ETS unit price of $25.
Transpower says it will probably be a couple of years before it needs to purchase any more SF6.
Chief executive, Patrick Strange, says the operator has a responsibility to keep its costs down, and if it didn't buy the gas now, the increased costs would be passed on to customers.
Green Party climate change spokesperson Kennedy Graham says Transpower's behaviour nullifies the whole purpose of the ETS.
He says the Government should order an investigation into Transpower's behaviour.
Climate Change Minister Tim Groser had not responded to calls from Radio New Zealand.