The self-proclaimed champion of the regions - Shane Jones - is being accused of over-inflating the climate change benefits of his One Billion Trees programme.
The Regional Economic Development Minister said planting the trees will create economic, social and environmental benefits, but National believes he has over-egged the wins.
The government aims to plant a billion trees by 2028.
Mr Jones said the programme is well on its way to the goal, with this year's target exceeded by more than 60 million trees planted.
It's hoped the tree planting programme will help reduce New Zealand's climate change liability under the Paris Agreement - it's estimated it could cost the country about $32 billion in 2030.
Mr Jones told Parliament in August that the billion trees programme would shave about three billion dollars off that bill, but National's economic development spokesperson Paul Goldsmith said Mr Jones had inflated the benefits.
"We checked with the officials to get a breakdown of that figure, and it shows that he was exaggerating, and way off.
"The officials best estimate is that the billion trees programme might reduce our liabilities by $900 million, so less than a third of what Shane Jones referred to," Mr Goldsmith said.
Documents released to National under the Official Information Act show that overall, tree planting by 2030 should reduce the Paris Agreement liability by $2.7b, but that only $900 million of that came from the One Billion Trees programme - the rest was from proposed changes to the Emissions Trading Scheme.
Paul Goldsmith said even the $900 million figure was based on the assumption that New Zealand would write out a cheque for $32 billion in 2030 when the Paris Agreement wash-up took place.
"There is a huge amount of water that needs to flow through the bridge, so we've been criticised for asking too many of Shane Jones, and the point is that when we do ask the questions more often than not we find that what he has said is exaggerated and quite incorrect."
Shane Jones, in his usual fashion, is completely unfazed by Mr Goldsmith's arguments.
"Well Goldy has a passion for being pedantic and he's taken it to a whole new rats and mice level now."
Mr Jones said it was important to remember that the brand that the government was pursuing in terms of the billion trees, was made up of a number of things.
"Not only seedlings, not only the improvements to the ETS which are yet to be made public, and the improvements to the foreign direct investment regime, which provides a primrose path for forestry, and the $240-odd million for grants, planting and when you add all of that up - that is what I am referring to."
Shane Jones said the entirety of all of those interventions was what made up the billion trees brand.
"Paul Goldsmith thinks he's found a snapper, but really it's just a sprat."