Provisions to rehabilitate the state's 450 mines in NSW are inadequate, their required outcomes vague and the risks of long-term failure are uncovered, according to a report by the Auditor-General.

While the total value of deposits made by mining companies had quadrupled since 2005 from $500 million to $2.2 billion as of the end of last year, the cost calculation used by the government had not been adjusted since 2013, the report found.

Security deposits "should cover the full costs of rehabilitation for that mine" but those held by the Department of Planning and the Environment "are not likely to be sufficient to cover the full costs of each mine's rehabilitation in the event of a default", it said.

"Some activities required for effective rehabilitation are not covered, or not covered adequately."