NSW's new biodiversity offsets scheme is likely to save developers such as coal miners millions of dollars, according to a leaked report commissioned by the Berejiklian government.

A cost benefit analysis by the Centre For International Economics, a copy of which was obtained by Fairfax Media, found Glencore's Mt Owen coal mine extension in the Hunter Valley would have reaped huge savings if the project had been assessed under the new methodology.

Mining trucks at Glencore's Mt Owen coal mine in the Hunter Valley. Credit:AFR

Using two methods to gauge the mine expansion's offset costs, the reforms to the scheme both show savings of "approximately $80 million" compared with existing compensation tally for "disturbing" an extra 485 hectares of native vegetation, the report found.

Similarly, the Dubbo Zirconia Project, a $1 billion venture due to "disturb" 815 hectares, would save the developers about $12.89 million because fewer offset credits would be needed.