Despite almost 60 per cent of NSW's mammal species and a third of the birds on the endangered list, the Berejiklian government is persisting with conservation schemes that amount to a "bad joke", critics say.

A report, titled Paradise Lost has found biodiversity offset schemes between 2005 and 2016have failed to deliver outcomes promised by developers of mines and other major projects.

Of eight case studies where the destruction of habitat was permitted in exchange for protection elsewhere, the results of two studies were found to be "adequate", and five others "poor", according to the Nature Conservation Council, which compiled the report. For the Boggabri/Maules Creek area, which has two huge open-cut coal mines, the outcome was found to be "disastrous".

In the latter case, miners will clear about 4000 hectares – more than half of the Leard State Forest, which is home to 36 threatened species including the diamond firetail and the masked owl – the report said. More than a quarter of this area is made up of critically endangered box-gum woodland.