Scientists are creating colonies of rock oysters around Sydney Harbour to help clean up the environment and boost biodiversity.

Ecologists from Macquarie University and the Sydney Institute of Marine Science are growing oysters on tiles placed along sea walls around Balmain and Waverton to mimic the complexity of a rocky shore.

At other sites, teams have been conducting similar trials with bags of oyster shells.

It is expected to encourage oysters to attach and form more habitats than they would have on existing flat sea walls which make up about 50 per cent of the harbour.

Sydney rock oysters will be grown on tiles to boost biodiversity in Sydney Harbour. ( Supplied: Reef Design Lab )

"In the last five years we've realised [oysters] were really abundant parts of our ecosystem and they're really not present like these used to be," marine ecologist Associate Professor Melanie Bishop told 702 ABC Sydney.

"We're dealing with a highly urbanised environment and trying to come up with innovative solutions that work with this urban context."

The research is part of the World Harbour Project aimed at restoring ecosystem function to urbanised estuaries.

The tiles are being added to 14 other harbours around the world including sites in Auckland, San Francisco, Hong Kong, Dublin, Taiwan, South Africa and Chile.

Associate Professor Bishop said it was important to enhance the ecological value of urban structures to bring marine life back into cities.

Melanie Bishop researches estuarine and coastal ecosystems that have bore the brunt of human impacts. ( Supplied: Alexander Edwards )

"On the land we have the green walls and the green roofs, but in the sea we're a little bit behind," she said.

"We're starting to think about similar concepts so how can we design sea walls, how can we design pontoons, pilings and piers to maximise their ecological value."

She said it was estimated that the eastern southern coast of Australia had lost 99 per cent of its oyster population since European settlement.

Oysters produce pseudo faeces

Oysters are filter feeders and have the capacity to filter half a litre of water an hour and take up metals present in the water.

Oysters provide homes for native flora and fauna such as seaweed, crabs and small fish. ( 702 ABC Sydney: Amanda Hoh )

They are regularly used as bio-indicators for metal pollution.

"What they're doing is filtering water to get phytoplankton and organic particles and food out of the water," Associate Professor Bishop said.

"What they do is produce pseudo faeces, like a type of poo, and they poo that out below and add nutrients into the sediment.

"In doing this, they're helping to maintain water quality by taking all those fine particles out of the water and making it clearer."

One problem for the environment is that metals stay in the oysters, causing them to bio-accumulate and be passed on to other species that feed on the oysters, Associate Professor Bishop said.

But while scientists could harvest the oysters, the benefits of leaving them in place would outweigh any potential issues.

While Sydney rock oysters are more familiar in NSW, angasi or flat oysters which are common in southern Australia are starting to be commercialised. ( Dr Chris Gillies )

"We see the main benefit of the oysters in maintaining water quality," she said.

"Maintaining clean water allows light to get through for things like sea grass which needs lots of light to grow and survive.

"Oysters provide food and habitat for little crabs and juvenile fish, so there's much greater benefits for leaving the oysters alive and in place."

Associate Professor Bishop said her students had conducted surveys along sea walls and found there were "20 times as many species" among oyster colonies than in areas without oysters.

While she welcomed the public to go and look at the colonies over the sea walls, she warned the creatures were not safe to pick and eat.

She said oysters served in restaurants or bought from supermarkets were subjected to tight health frameworks, water quality controls and filters to ensure they were safe to eat.