Tonkin, star of The Vampire Diaries television series, posted a selfie on Instagram of her wearing the bizarre-looking device.
The feature story said the All Blacks were among top sports teams wearing the masks, a fact confirmed by New Zealand Rugby.
The mask claims to 'trap and recycle your own moisture in your breath, ensuring that the air you breathe remains moist, thus preventing the dehydration that contributes to your jet lag and in-flight infections'.
'Ready #longflightahead #needsawinestraw,' the 27-year-old captioned her tongue-in-cheek photo, which quickly amassed more than 30,000 likes within an hour.
Ms Tonkin's fans, however, seemed to be a little confused by the unusual accessory.
'Sorry, does anyone know why she is wearing this?,' someone posted.
'Oooh, I need one of these,' another more enthusiastic person added.
According to the Humidiflyer website, the Irish national rugby team and the Australian swim team also use the filtered face mask on long-distance flights.
The site even says opera singers and businessmen and women are also users.
The contraption was originally designed by a Kiwi Paul Aberhart, who was an international flight attendant with Air New Zealand for 11 years.
Aberhart explained that he came up with the device after seeing how badly both he and the rest of the crew struggled with sinus problems on flights:
"This situation prompted me to think that there should be a better solution. The concept of the HumidiFlyer was born," he said.
A New Zealand Rugby spokesperson said the masks are designed to keep players "away from bugs in the plane" and was "part of a comprehensive plan to stay hydrated, sleep well and be in top shape on arrival".
The All Blacks also wear "skins" (compression clothing) and follow a strict rehydration plane during long haul flights such as the one to South Africa.
"It's all about getting that competitive edge," said the spokesperson.