The NSW government has been accused by the state opposition of launching "the single biggest attack" on the National Parks and Wildlife service since it was set up half a century ago.

Penny Sharpe, Labor's environment spokeswoman, used a NSW estimates hearing to press the government to confirm the loss of 49 park rangers – or 20 per cent of the total ranger staff – since the coalition came to office in 2011.

The Royal National Park, reserved in April 1879, was the world's second national park. Credit:Peter Rae

The upper house member also asked the government to explain how the NPWS had seen its budget outlay slide from a projected $503.1 million in 2016-17 to a revised $448.1 million actually spent. That tally edged lower to $445 million in the current budget.

Anthony Lean, the chief executive of the Office of Environment and Heritage, told estimates the "bulk" of the reduction in spending was the result of a transfer of corporate services to other units.