SYDNEY - New Zealand immigrants of a different kind are making their mark in Australia.

At last week's Molluscs 2000 conference in Sydney, delegates were concerned at the spread of the New Zealand screw shell, Maoricolpus roseus. Introduced to Tasmania about 70 years ago, the species has now appeared in Sydney Harbour.

"It's a millennium marathon with uncertain environmental consequences," says an ecologist at CSIRO Marine Research, Dr Nic Bax.

He estimates the species now covers an area of sea floor about the size of Tasmania.

The filter-feeding mollusc has established itself in vast beds in northern Bass Strait and off the coasts of eastern Tasmania, Victoria and New South Wales.

It is not clear how it arrived in Tasmania, but many think it came in the dry ballast of stones gathered in New Zealand. Another possible source was the transtasman live oyster trade, which was common at the time.

Scientists are concerned about the screw shell's impact on other mollusc species, including scallops and the native Australian screw shell.

It seems the Kiwi variety is a particularly tough nut to crack for eating and has few predators. This affects shellfish-eating fish species, and the food chain that depends on them.

Dr Bax says the New Zealand screw shell threatens other species because it breeds so prolifically, smothering the sea floor to a depth of 80m along the Continental Shelf.

The screw shell is one of an estimated 200 marine species intentionally or accidentally introduced to Australian waters since the start of trade and shipping.

"We are only just starting to understand the habits of this species," says Dr Bax.

"It is highly competitive with other species, and builds substantial beds to the detriment of other animals on the sea floor.

"The concern is not simply some economic impact down the track.

"We don't know yet how far north it may travel, nor do we know how much it will alter the existing natural habitat and shut down the habitat of other species."

Dr Winston Ponder, of the Australian Museum, believes the mollusc is likely to displace similar, related species of screw shells, several of which occupy the same depth range and sediment profile as the Maoricolpus.

Researchers are working with fishers to build a profile of the screw shell's spread.