Analysts expect 12 million to 13 million phones to fly off the shelves in the first weekend, up from more than 10 million last year when the hugely successful iPhone 6's launch was delayed in China, the world's biggest smartphone market.
Among the first to pick up the new iPhone 6s in a cold, rainy Sydney was a telepresence robot named Lucy, operated by marketing executive Lucy Kelly.
"I obviously have my work and other things to attend to and can't spend two days lining up so my boss at work suggested I take one of the robots down and use it to stand in my place," she said via an iPad mounted on top of the wheeled robot.
"I love new gadgets. The new camera is meant to be amazing."
Fans from San Francisco to London to Sydney have camped out for days prior to the release, and Apple said earlier this month that pre-orders suggested sales were on pace to beat last year's first-weekend performance.
Apple's flagship iPhones generated nearly two-thirds of the US giant's revenue in the latest quarter. First released in 2007, the iPhone is Apple's best-selling device to date.
"The stage is set for Apple to show year-over-year growth over the Herculean iPhone 6 sales," FBR Capital Markets senior analyst Daniel Ives said.
After a dramatic redesign last year in which Apple enlarged the iPhone's screen and added mobile payments, the iPhone 6s and 6s Plus boast more modest improvements.
The phones, which are the same size as last year's models, feature improved cameras and 3D touch, a display technology that responds according to how hard users press their screens.
"Today is like Christmas for pocket film makers all around the world because the iPhone 6s Plus is like the newest, greatest toy we have to play with," said Jason van Genderen, who makes movies on smart phones in Sydney.
"I've never seen anything like it, it's astounding. The camera craft has now come up to story telling craft."
Apple executives have said just a fraction of their customers have upgraded to the iPhone 6, suggesting they have plenty of room to grow this year.
Lackluster offerings this year from rival smartphone manufacturer Samsung Electronics Co Ltd also will help Apple stand out in the marketplace, analyst Patrick Moorhead of Moor Insights & Strategy wrote in an email.
"Over the long haul, the 6s will eclipse the 6 as Apple is even more competitive versus Samsung in emerging regions and is gaining share in traditional regions," Moorhead wrote in an email.
"Samsung didn't bring a whole lot of compelling features to consumers with their new lines of phones."
The iPhone 6s and 6s Plus, which start at $199 and $299 (roughly Rs. 13,100 to Rs. 19,700) with a two-year contract with a mobile service provider, go on sale on Friday in Australia, Canada, China, France, Germany, Hong Kong, Japan, New Zealand, Puerto Rico, Singapore, the United Kingdom and the United States.
© Thomson Reuters 2015