The Great Barrier Reef

A global study of reef marine parks identifies what's needed to make them successful. Photo: AP

The O'Farrell government's decision to allow recreational fishing in NSW marine parks has undermined decades of research as well as harmed the region's ecosystems, according to a leading marine scientist.

Last March, the government offered a temporary "amnesty" to shore-based anglers in six marine parks that had previously banned fishing. It is now considering whether to make the change permanent.

That decision had "contaminated our sampling program", said Professor Graham Edgar, from the University of Tasmania's Institute for Marine and Antarctic Studies.

He cited the disturbance of work at Jervis Bay, where monitoring since 1996 was now of limited use.

Politicians often complain about the lack of good data, but "at the wave of a pen, or an announcement, people are now fishing in our monitoring sites, so it's back to square one", he said.

Professor Edgar is also lead author of a report published on Thursday in the British journal Nature, that highlighted the value of preserving no-take marine protected areas around the world.

Analysing data from 1997 to mid-2013 gathered in 87 reef marine parks in 40 nations, the research found many protected areas were often of similar health to adjacent fished areas. Key measures needed to ensure the parks' success in achieving conservation goals include well-enforced no-take policies, the paper found.

The healthiest parks were also typically large, at least 10 years old, and isolated from fishing zones by deep water and sand.

When the main criteria were met, marine parks had on average eight times more large fish and 14 times more sharks than fished areas, according to the study, which took in volunteer and expert researchers from countries ranging from New Zealand and Tanzania to Brazil and Italy.

Professor Edgar, who was a commercial fisherman in his teens on the NSW central coast, said the need for protected areas had never been greater as human pressures on marine environments increase, with the effects often little noticed and poorly understood.

"There are big changes that have been happening in the underwater world but they're out of sight," he said.

"If they'd happened in the terrestrial world, they'd be a lot of action."