The 32-year-old star was reported to have been flown by helicopter for treatment in Australia after contracting a bacterial infection - possibly typhoid - while shooting his adventure series Killing Zac Efron.
In a tweet on Monday, the Bad Neighbours star said he was back home "for the holidays with my friends and family" - and the post was accompanied by a photo of him with locals in Papua New Guinea.
He added: "Very thankful to everyone who has reached out. I did get sick in Papua New Guinea but I bounced back quick and finished an amazing 3 weeks in P.N.G."
Glenn McKay, a doctor with the Medical Rescue Group, told Sydney's Daily Telegraph newspaper on Sunday that he could not discuss confidential patient information, but could confirm "that Medical Rescue retrieved a US citizen in his 30s from PNG to Brisbane recently for medical attention".
The newspaper reported that doctors allowed Efron to fly home to Los Angeles on Christmas Eve.
Typhoid fever is a bacterial infection that is transmitted by contaminated food and water, and kills 216,000 to 600,000 people worldwide each year.
Killing Zac Efron is a survival reality show in which the actor ventures "deep into the jungles of a remote, dangerous island to carve his own name in expedition history".
The series was commissioned by short content platform Quibi and is set to launch in April.
Efron, who found fame in Disney's High School Musical franchise, had previously posted images on social media showing him in a canoe on PNG's Sepik River and travelling to Yanchan Village to see a traditional skin-cutting ceremony.
Source: Sky News