The Public Service Association, which represents park rangers, has ordered its members not to assist in establishing recreational hunting in national parks in NSW.

The association's general secretary, John Cahill, says without the support of park rangers, a plan to introduce regulated hunting under the supervision of NSW National Parks would likely fail.

He called on uniformed rangers and other union members to attend a rally organised by the Greens tomorrow outside Parliament House.

The Shooters' Party, which holds the balance of power in the Legislative Council, says the bill will allow licensed gun owners the opportunity to play a more active role in conservation hunting programs in public lands.

The upper house MP Robert Brown says the intention is only to kill feral animals, not native.

But Mr Cahill said recreational shooters would compromise the feral animal control programs run by national parks officials, putting native flora and fauna at risk.

He expressed concern about danger to park rangers and the public if shooting were allowed in bushland popular with hikers and picnic groups.

The cosy relationship between the Government and the Shooters' Party disintegrated over a private member's bill that would allow hunting of feral animals in national parks and the establishment of game reserves.

Since mid year the Government has had to negotiate with the Opposition and the Greens to get legislation through the upper house.

Mr Cahill said any deal on national parks is "an attempt to garner the support of the Shooters' Party to assist in passing legislation through State Parliament".

The Greens MP Lee Rhiannon said: "For the PSA to break ranks … shows how universally repugnant shooting in national parks is and how low the Government's reputation has sunk."