The offending garment?

A black and white T-shirt sporting a smiling koala on the front, under the words 'Save our south coast koala'. On the back, it read 'save me, save my forest'.

"I've worn this T-shirt as recently as March this year into Parliament, but this time security have obviously been given some sort of firmer instruction," said Mr Plumb from East Ryde.

He had planned to visit Parliament House to support the launch of a report by leading conservation groups, including the Nature Conservation Council, a group which he supports.

The New Deal For Nature report outlines 10 essential components that conservation groups say must be included in the state government's controversial new Biodiversity Conservation Act.

"I came along to support it. These days I wear a koala T-shirt quite constantly, because I am a campaigner on it," said Mr Plumb, who is the convenor of the forest and koalas conservation group ChipBusters.

"We see major populations of koalas close to extinction. I didn't think [the T-shirt] would be an issue at all, but I was stopped at the entrance."

A spokesperson for state parliament said a long-standing "demonstrations policy" was in place, which ensured the orderly operation of Parliament is not disrupted by public demonstrations, protests and publicity stunts.

"Under the policy, it is a condition of entry that banners, signs, T-shirts or any item bearing demonstration related-branding are not permitted in the parliamentary precincts. Parliament does not provide commentary on specific incidents," she said.

Mr Plumb said he did not want to detract from the launch of the report, but he felt he could not ignore what was a "very anti-democratic authoritarian situation".

The NSW Government is currently rewriting the state's conservation laws, and the New Deal For Nature report calls into sharp relief the need to address more than 989 plant and animal species being threatened with extinction in NSW.

It was prepared as a joint venture by the Nature Conservation Council of NSW, WWF-Australia, National Parks Association of NSW, Humane Society International, The Wilderness Society, Total Environment Centre and Colong Foundation for Wilderness.

"Every country has its icons. For us, the koala holds a special place in our culture," the report says.

"Yet if current trends continue, koalas may vanish from much of New South Wales. Their numbers have reduced by more than a third over the last 20 years alone. The primary cause is the destruction of their habitat – the forests and woodlands of our great state."

Thursday's launch was widely attended, with "standing-room only". Among those in attendance were 10 MPs representing all of the major parties.

Upper House Liberal MP Greg Pearce accepted the report on behalf of the government, saying he would hand it on to Environment Minister Mark Speakman.