After spending six weeks in hospital being treated for anaemia, parasites on his skin and a head injury, Jervis, a 100-kilogram green turtle, has been released into Sydney Harbour.

The reptile was found washed up at a beach on NSW's South Coast in May.

He spent a month and a half recovering at Taronga Wildlife Hospital and was released just outside Sydney Heads on Thursday.

"He was very sick," Taronga senior veterinarian Dr Larry Vogelnest said.

"But thankfully he recovered quickly."