About this time of year, birdwatchers start looking into the sky for Swifts. They're relatively common up North in the tropics, but down here south in Melbourne, they're a bit of a rarity.
There are two common types, which used to be known as the Fork-tailed Swift and the Spine-tailed Swift. Taxonomists are nearly as bad as astronomers, though, and so they had to change their names - to Pacific Swift and the White-throated Needletail, respectively.
The easiest way to tell them apart is by looking at their underparts (that's what you're going to see as they fly high above you). The Needletail's neck and rump are white, whereas the Pacific Swift is more uniform underneath, with a dark rump and maybe a slightly lighter chin and neck.
Although they do land occasionally, Swifts spend most of their time on the wing. I understand that they even sleep in the air, taking little naps while on autopilot.
Their food consists of insects that are stirred up by weather fronts or bushfires, which means they're often seen just before storm fronts or other weather changes.
Their wings have a heavily swept-back aspect, much more than Swallows, which are close relatives. So, to see them, wait for the weather to change, and just before the storm hits, look high in the sky for C-shaped birds. You'll need a pair of binoculars, preferably 8 or 10 power, and a steady hand.
If you do see them, send an email to the Birding-Aus newsgroup (they also have a Facebook page). Swifts are rare enough to be this interesting.