Happy Feet, the lost penguin who became a worldwide celebrity after he washed up on a New Zealand beach was released back into the Southern Ocean on Sunday to begin a long swim home to Antarctica.

"It's an indescribable feeling to see a patient finally set free. It?s definitely the best part of the job," said veterinary surgeon Lisa Argilla who treated the penguin after he was found emaciated and near death in late June.

The emperor penguin, dubbed Happy Feet, was released into the water from the New Zealand fisheries vessel Tangaroa near Campbell Island, about 700 kilometres (435 miles) south of New Zealand's South Island.

His home in Antarctica is about 2,000 kilometres further south and it is expected he will join up with other emperor penguins on the long voyage.

Argilla, the manager of veterinary science at Wellington Zoo, said in a statement from the Tangaroa that the penguin was released down a purpose-built hydro-slide after other options were abandoned because of rough seas.

He needed "some gentle encouragement" to leave the safety of his crate that has been his home for six days since leaving New Zealand after spending two months at Wellington Zoo, she said.

"He slid down his specially designed penguin slide backwards but once he hit the water he spared no time in diving off away from the boat."

The penguin had travelled south in a custom-made crate designed to keep him cold and comfortable during the voyage.

Happy Feet, found on a beach near Wellington in mid-June, was only the second emperor penguin ever recorded in New Zealand.

He was close to death and needed surgery to remove sand and sticks from his stomach before he could be fattened up on a diet of fish milkshakes.

The three-and-a-half-year-old bird, which now weighs about 27.5 kilograms (60.5 pounds), attracted international attention during his New Zealand sojourn and there are plans for a book and documentary recounting his story.

Attendance at Wellington Zoo almost doubled during Happy Feet's stay, even though he was rarely on display. His fans include New Zealand Prime Minister John Key and actor Stephen Fry, who is in Wellington to film "The Hobbit".

He has been fitted with a satellite tracker and microchip and his progress can be followed on www.nzemperor.com.