Forest and Bird has told a court that Buller Coal may have overestimated the number of jobs a proposed mine on the West Coast's Denniston Plateau could produce.
The conservation group's appeal against an interim decision from the Environment Court is being heard at the High Court in Christchurch.
In its interim decision in April this year, the Environment Court signalled it would rule in favour allowing Bathurst Resources consents to mine - as long as it had more clarity on issues such as rehabilitation of the conservation land.
Last week, Conservation Minister Nick Smith gave Bathurst access to Department of Conservation land, but the company still requires a resource consent before it can begin work.
Forest and Bird told the court there was a difference of 1 million tonnes in two forecasts Buller Coal Ltd had for how much coal the mine could produce.
Buller Coal Ltd owns and operates all of Bathurst's operations on the Buller coal field.
Forest and Bird said there was a direct correlation between this and the number of people the mine could employ.
Forest and Bird's lawyer said this was important because in making its decision, the Environment Court had to weigh up the good the mine would do in creating jobs against the amount of environmental damage it would cause.
However, Justice Fogarty said there was nothing unusual in there being two different forecasts for how much coal the mine could produce.
The appeal began on Monday and is set down for three days this week.
