Land-clearing approvals in New South Wales have increased nearly 13-fold since the Coalition government relaxed laws in 2016, according to a secret report to the state cabinet by its Natural Resources Commission.

The report, marked “Cabinet in Confidence”, was commissioned by the government in January 2019 under an agreement between the Liberals and Nationals to review land clearing if applications exceeded 20,000ha a year. The commission handed it to the government in July, but released it only after the Independent MP Justin Field threatened legal action.

Field said it was evidence that land management under the NSW Nationals was a catastrophic failure. He called for clearing approvals to be paused until steps were taken to prevent agriculture and forestry exacerbating the biodiversity loss from the unprecedented recent bushfires and drought.

The commission found more than 37,000ha were approved to be cleared last financial year, almost 13 times greater than the annual average rate across the decade to 2016-17. Approvals jumped more than 70% after the rules covering land clearing changed at the start of 2019, rising from 25,247ha in the final quarter of 2018 to 43,553ha in the first three months of the new year.

The commission found the extent of the land clearing and what is described as “thinning for pasture expansion” was putting the state’s biodiversity at risk. The government had promised to protect between two and four times as much land as it cleared, but had failed to do that in the majority of the state.

It also highlighted the lack of an effective monitoring and compliance regime to ensure laws were enforced. In a six-month stretch between August 2017 and January 2018 there was 7,100ha of unexplained land clearing. It was 60% of the clearing in that time.

Field said it was “unacceptable and unforgivable” that the government had held back a report that showed land clearing was continuing at record rates while posing a risk to biodiversity across the state. “I’m concerned that, in the current Covid-19 crisis, unacceptable rates of land clearing are going to continue unabated and without enforcement,” he said.

The Nature Conservation Council of NSW said the report showed the National party was incompetent. Its chief executive, Chris Gambian, said it was a damning assessment of how the government had handled what was supposed to be a signature reform.

“This report is alarming because land clearing is a key threat pushing most of the state’s threatened species towards extinction,” he said.

“Koalas and other vulnerable species are being smashed from every direction, by bushfires, drought, logging and land clearing. Land clearing is one of the few threats we can tackle directly, but the National party is preventing this government from doing what is needed.”

Gambian called on the government to release regulatory maps that were still not available two years after promised.

A NSW government spokesman said it had received the commission’s advice, was considering it carefully and would respond to its recommendations.

Land clearing also contributes to the climate crisis. The Guardian last year showed the increased pace of clearing in several states, including NSW, meant just two years of bulldozing would effectively cancel out more than $1.5bn of taxpayer funded climate change projects paid for through the federal government’s emissions reduction fund.