Apple has three new iPhones. Here's everything you need to know about them.

-With AAP

Apple has unveiled three new iPhones, including its biggest and most expensive model yet, as the company seeks to widen the product’s appeal amid slowing sales.

Apple also said that its next major update to its iOS operating system will come next Tuesday, followed a week later by a Mac software update.

Here are the new phones and what you can expect from them:

iPhone Xs

CEO Tim Cook on Wednesday showed off the Apple Xs, which has a 5.8-inch screen, bigger than the one on last year’s dramatically designed model, the iPhone X.

It’ll cost $1,629 in Australia and be available in silver, gold and space grey.

The iPhone Xs camera has new dual 12-MP sensors, one for wide-angle shots and one for telephoto. When you want to shoot things in portrait mode or with special depth effects, both cameras work together to give your photos extra dimension.

Apple’s new smart HDR feature will shoot a four-shot burst, meaning multiple frames can be taken at once and the processor will then merge them into one great photo.

iPhone Xs reaches a new level of splash and water resistance for up to two metres for 30 minutes and protect against everyday spills including coffee, tea and soft drink.

Pre-orders start on September 14 and the phone will ship on September 21.

iPhone Xs Max

A bigger version will be called the iPhone Xs Max, which looks to be about the size of the iPhone 8 Plus, though the screen size is much larger at 6.5-inches. This one will cost $1,799 in Australia.

As with the iPhone X, the new phone has a screen that runs from edge to edge, an effort to maximise the display without making the phone too awkward to hold. The screen needs no backlight, so black would appear as truly black rather than simply dark.

Like the iPhone Xs, the iPhone Xs Max is water resistant.

This even-bigger iPhone, which will be available on September 21 (with orders open the week before) represents Apple’s attempt to feed consumers’ appetite for increasingly larger screens as they rely on smartphones to watch and record video, as well as take photos wherever they are.

iPhone XR

Apple also showed off a cheaper iPhone, called the iPhone XR. This phone integrates technologies from iPhone XS in an all-screen glass and aluminum design with the most advanced LCD in a smartphone.

The iPhone XR is available in six colours - black, white, blue, yellow, coral and red.  A portion of the sales from the red model will go Global Fund HIV/AIDS grants that provide testing, counseling, treatment and prevention programs with a specific focus on eliminating transmission of the virus from mothers to their babies.

It has a traditional, lower-quality screen and an aluminium body; it's physically smaller than the iPhone 8 Plus but has a bigger 6.1-inch screen and is water resistant.

Unlike last year's iPhone 8 model, the iPhone XR has Apple's Portrait Mode for super sharp selfies and Face ID for secure facial authentication.

iPhone XR features a 12-megapixel wide-angle lens with an all-new sensor, delivering faster auto-focus, while larger and deeper pixels improve image fidelity and low-light performance on photos and videos.

The iPhone XR will have no home button, but it will vibrate to simulate a button press.

It'll cost $1,229 in Australia and come out on October 26.

Apple Watch

The Apple Watch is also getting an update.

Apple announced updates that push its Apple Watch further into medical device territory. It has a larger screen and a built-in heart sensor that the company said can detect irregular heart rates and perform an electrocardiogram. The latter feature has been cleared by the US Food and Drug Administration, the company said.

These features will be available to US customers later this year, but Apple did not say when it would make it to the rest of the world.

In addition, Apple says the Series 4 Apple Watch will also be able to detect when someone falls - and can tell the difference between a trip and a fall. If it detects a fall and the user doesn't respond in a minute, it'll automatically call for help. This feature may be especially attractive to older people or those with elderly parents worried about falling when no one is around to help.