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The 32-year-old was reportedly flown by a medical evacuation crew from Papua New Guinea to Australia, where he underwent emergency treatment.
It's understood he had contracted a 'form of typhoid or similar bacterial infection' which required urgent care.
Dr Glenn McKay, director of Medical Rescue, the organisation that oversaw Efron's flight, would not discuss a specific patient's case. However, he did confirm to the Telegraph that they 'retrieved a US citizen in his 30s from PNG to Brisbane for medical attention in Australia'.
The actor was said to have been in a 'stable' condition when he was admitted to St Andrews War Memorial Hospital, Brisbane, where he stayed for seven days before flying back to the United States on Christmas Eve.
The High School Musical actor was in Papua New Guinea to start filming for Killing Zac Efron.
The new series will follow Efron as he delves deep into 'the jungle of a remote, dangerous island, falling off the grid for 21 days', according to the Hollywood Reporter.
After landing in the country, Wewak tour guide Cyril Tara posted a bunch of photos with the actor as he showed him around.
On 7 December, he shared a photograph of the pair together, with the caption: "I am very happy to be his guide."
The series is being made by online platform Quibi, with Efron named as the executive producer.
Over the course of the series, he will throw away the luxuries he has become accustomed to, instead trying to survive with nothing but a few basic items and the help of his guide.
Speaking ahead of filming, the American said: "I tend to thrive under extreme circumstances and seek out opportunities that challenge me on every level.
"I am excited to explore any uncharted territory and discover what unexpected adventure awaits!"
Entitled Scoob, the new movie could hit screens around the world as early as May 2020. Alongside Efron, who plays Freddie, we'll see Will Forte as Shaggy, Gina Rodriguez as Velma, and Tracy Morgan as Captain Caveman.
LADbible has contacted Zac Efron's representatives and Quibi for comment.
Featured Image Credit: Facebook/Cyril Tara
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