A southern elephant seal pup brought back from the brink of starvation has been returned to the wild, after large doses of fish smoothies restored his health.

The malnourished pup, which became known as Elmo, washed up on the foreshore at Port MacDonnell in the south-east of South Australia about two months ago.

Weighing less than half its expected body weight the pup, which was estimated to be between six months and a year old, was taken to Torrens Island in Adelaide and nursed back to health by volunteers from the Australian Marine Wildlife Research and Rescue Organisation.

Its president Aaron Machado was part of a team which took Elmo back to the south-east for an early-morning release.

Having increased the seal's weight from 55 kilograms to more than 90, Mr Machado said Elmo was back where he should be.

"We slowly increased his diet to where he was having one litre of fish smoothie a day, that's 450 grams of fish, water and oils."

Mr Machado said the return to the sea went smoothly too.

"That was a textbook release. He came out [of the trailer], got his bearings and now he is where he should be," he said.

Elmo was released at Brown's Bay near Port MacDonnell and it was thought likely he would swim to remote Macquarie Island.

He now has a microchip so his movement can be monitored.