Happy Feet the rescued penguin has vanished in the ocean on his way home from New Zealand just days after his release, leaving behind a mystery that may never be solved.

After gaining global celebrity by going off-course and landing on a New Zealand beach far from its Antarctic abode, the emperor penguin has simply disappeared.

Happy Feet's satellite transmitter went silent last Friday, five days after experts released the bird from a research ship into the Southern Ocean about a quarter of the way down to Antarctica.

Initial dispatches from the device showed that Happy Feet swam in a meandering route, ending up about 75 miles south-east of where he began by the time the last transmission came across. Experts say his looping pattern was typical for a penguin chasing fish.

At this point, the transmitter may have simply fallen off, experts said. It was attached to the bird's feathers with super glue and was supposed to fall off anyway early next year when he moulted.

"Who knows? He's probably swimming along quite happily without a transmitter on his back," said Peter Simpson, from New Zealand's department of conservation.

Or, he may have died of natural causes.

Or, something more sinister: Happy Feet could have been eaten by an orca or a leopard seal.

There is a small chance the truth could be revealed one day because of another, small device implanted under the bird's skin. This transponder chip could send a signal if it comes close enough to an Antarctic monitoring site, but that might take years.

Named after the penguin odyssey film "Happy Feet," the bird was discovered on the beach in June.

He became sick from eating sand which he mistook for snow, but was nursed back to health over two months at the Wellington Zoo. Vets repeatedly flushed his stomach to remove sand and fattened him up on a diet of fish before he was released back into the ocean on September 4.