UNDERWATER surveys have begun to determine the cumulative impact of dredging and dumping, following criticism from green groups that the Government was ignoring the problem. The Civil Engineering Department spent $1 million last month photographing more than 400 local underwater sites with remote cameras, and will repeat the exercise every three months. The dredging and dumping has destroyed coral and is feared to be driving fish away. Green groups welcomed the underwater surveys, but remained sceptical they were being carried out too late. Lisa Hopkinson, of Friends of the Earth, said: ''Of course it is useful - the clearer the picture you've got of what's happening on the seabed, the better - but there has to be some purpose for the monitoring. Otherwise it's an historical archive.'' Jo Ruxton, of the World Wide Fund for Nature, said: ''I can't help thinking it's all come a bit late. What's the point of finding out what's there as they destroy it?'' John Massey, who heads the Civil Engineering Department's fill management committee, revealed the surveys were in response to concern that the Government was monitoring only individual dredging sites and not the territory-wide impact. He agreed the work ideally should have been done before dredging started, but said the dredging work made money available to fund the surveys. ''If you looked anywhere else, you would find that you don't get detailed specific documentation until there's a need for it. And the people who tend to do that documentation are the proponents of dredging,'' he said. ''$1 million is a lot in terms of a research grant to a post-graduate student at university, but not in terms of the billions of dollars being spent on dredging.'' Mr Massey said the information was not merely a log of what was being dredged. The surveys covered not only dredging and dumping areas, but relatively untouched sites such as Mirs Bay. ''This can remind people that the marine environment is not being totally destroyed, and enable informed judgments about future dredging proposals,'' he said. ''It should also give a good idea of the effects of dredging and may indicate how we can speed up or improve recovery.'' But Ms Ruxton, a diver, said: ''We're actually observing the destruction of corals. If we can see it so clearly, what effect is it having on the fish and the food chain?'' The remote cameras pick out where sediment has settled on the seabed, what animals and other marine life are living there and how they are coping. Sediment samples are also taken at some sites to supplement the photos and dives are being made around the coastal seabeds that are hard to reach by camera. Dives made in August off the Ninepin Islands, where coral was destroyed by dredgers in 1991 before it was realised what was there, showed some of the coral may be re-generating. But Ms Ruxton believed re-growth did not mean the site was returning to its original condition. ''Some opportunistic species may come back, but you won't get the same diversity or the same amount of food that you had before,'' she said. She added that coral at Hoi Ha Wan, which was smothered by run-off after a hill was carved up for reclamation work in the early 1980s, had failed to recover.