The Duo's reception could prove an early test for new CEO John Ternus, the former hardware chief who unveiled the phone barely a week after taking charge in September.
Within minutes of pre-orders opening last week, Apple's estimated delivery dates for the Duo had slipped to late January as demand outpaced supply. The US tech company called the response from customers "nothing short of extraordinary."
In Sydney, the first Apple store to sell the phones on Friday, a queue of customers formed outside before opening at 8am AEDT.
Simon Krishnan, a 48-year-old engineer, paid $A3899 for a Duo with 512GB memory.
"This is a bit of a luxury item for me, because it's so expensive, but I just think, you know what? Life's too short," he said.
Early reviews have praised the Duo's design as well as its sturdy and barely visible hinge, a major feat for phones whose prominent creased screen has long been a drawback and potential deterrent for buyers.
But reviewers have said camera compromises including the absence of a telephoto lens and third-party apps that still need to be reworked for the new display size could disappoint some buyers.
They have also said the Duo in its smaller, folded state is less handy for the litany of quick tasks that most users carry out each day on standard iPhones, with several prominent reviewers recommending against the Duo for many Apple customers.
"It's been a lot of fun trying the Duo out for a week, but it feels like I've gotten to use an extraordinarily polished prototype — not a finished product that I'd want to buy and carry," longtime Apple commentator John Gruber, author of the blog Daring Fireball, wrote in a review.
The limited supplies could disappoint Apple fans lining up in front of stores to buy the Duo.
Apple told Reuters that walk-in availability would vary by country, with limited stocks in the US, Australia, Canada, Mexico and much of Britain and Europe. In China, France, Hong Kong, India and Japan, units would be available only for customers who had ordered online.
"Foldables have been an established category for years, but adoption has remained relatively limited compared with conventional smartphones. Apple's entry could bring the format to a much broader audience," said Francisco Jeronimo, vice president at market researcher IDC.
IDC expects Apple will take 44 per cent of the foldable value in 2026 on 31 per cent of foldable units. By 2027, it will become the market leader in the category both in value and units shipped, IDC forecast.
"Apple has made a convincing case for this form factor," Jeronimo said after spending a week with the passport-shaped Duo.
"Now comes the next test: turning excitement into availability, adoption and a lasting new chapter for the smartphone market."
