Satellite tracking shows the turtles frequent busy waterways including the harbour and Parramatta River, around Wollongong Harbour, Brisbane Waters near Gosford and up the Hawkesbury River as far as Cottage Point.
It is not known why the turtles spend so much time hugging the coast - when other species and older green turtles opt for the open ocean - but it places them at much higher risk from plastics and other marine debris.
The Taronga Wildlife Hospital tracked three turtles after nursing them back from serious injuries - observing one reach Longueville in upper Sydney Harbour, while the others travelled to Lake Macquarie and north to the busy tourist destination of Port Stephens.
Libby Hall, coordinator of the hospital's rescue and rehabilitation centre, said the study is plugging holes in the previously unknown behaviour of juvenile turtles.
"We know what the adults do because there's research all around the world ... They go up on their nesting beaches," she said.
"What we don't know is what they do in between those years. It's called the lost years.
"We thought they'd swim off into the ocean and we'd track them across the ocean, but no - the (juvenile) green turtles hug the coast."
Researchers are still attempting to discover why younger turtles spend as much time as they do in the far busier and dirtier waters, but Ms Hall thinks food sources might be the answer.
"They don't start eating seagrass until they're big adult animals, so they're probably feeding on jellyfish," Ms Hall said.
The behaviour is also unique to the green turtles, with other species, like hawksbill turtles and loggerheads making a beeline for the open ocean, with the tracking technology losing sight of them beyond New Caledonia and north of New Zealand.
One of three turtles currently undergoing treatment at the Hospital is Tama, who was brought in as a hatchling roughly 18 months ago after being found at Tamarama beach.
"She pooed plastic for six days, tiny pieces of plastic," Ms Hall said.
"She's now a good size and age where she can be released back into the wild."
The program, sponsored by waste-management giant Veolia, has tracked 41 turtles since 2014.
A satellite tracker is attached to the shell of a soon-to-be-released green sea turtle. (HANDOUT/TARONGA CONSERVATION SOCIETY AUSTRALIA)
WHAT TO DO IF YOU FIND AN INJURED TURTLE
* Keep noise to a minimum and don't let anyone touch the turtle unnecessarily
* Lift the turtle onto a blanket or into a plastic tub for transport
* Cover any injuries with a clean material so towels or straps used to cover the turtle do not make the injuries worse.
* Cover the head with a small towel but ensure the nostrils are not obstructed
* Do not remove barnacles as this causes the turtles pain and can lead to later infection.
* Cover the transport box with a towel and transport turtles inside the cabin of a vehicle.
* Contact your local vet or Taronga Wildlife Hospital on (02) 9969 2777
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