GRAZING cattle in NSW national parks will open them up to greater management by volunteers with vested interests, a union fears.

The trial was announced earlier this week in river red gum and cypress forests in the Riverina area.

Existing grazing permits will be extended to 2016 and no new land opened up for the scientific trial, the government said.

Today the union representing park rangers joined Labor and the Greens in criticising the plan.

The rangers likened the government's trials in NSW parks to Japan's scientific whaling program.

The Public Service Association said the introduction of "inappropriate" cattle grazing was an additional responsibility for rangers at the same time the government had cut National Parks staff.

"The cattle grazing trial is an ill-disguised foothold for more widespread cattle grazing across the state," PSA assistant secretary Steve Turner said.

"It will run down natural values and generate further excuses for 'volunteer' help from vested interests rather than proper resources for professional staff to get on with their job."

Opposition environment spokesman Luke Foley said the government had given in to the demands of the National Party and farmers, and grazing would "have a devastating impact on our native animals and plants".

Federal Environmental Minister Tony Burke told Weekly Times Now yesterday he believed national parks were for native species and weren't farms.

"I will have to check what legal powers I have; the legal authority I had in Victoria was a bit different because they were doing it at a site that had a national heritage listing on it which gave me wider ambit of legal power," Mr Burke said.

"But the principle of this is really simple. I believe national parks are there for our native species. I don't believe national parks are there for people to be using them as a site that ordinarily would be how you use a farm."