Actually, not strictly leafless as such. Casurinas have needles some ten to twenty centimeters long, a few milimeters thick. The needles are marked with fine rings every few centimeters. While admittedly most of the photosynthesis is carried out in the elongated leaf stalk that forms the body of the needle, there are rows of small, triangular leaves in the rings of the needle, through which gaseous exchange for the plant takes place.
Additionally, the varieties common around Sydney have long needles that stand upright rather than drooping. They have extremely shallow root systems ideally suited for the sandy soil around Sydney, and tend to grow in coastal areas or along the banks of rivers, but were planted in vast numbers throughout the city as fast growing native screen trees in the leadup to the 2000 Olympic Games. They grow quickly, with immature trees under ten years old several meters high but with a thin trunk. Older trees become more substantial, with trunks a foot thick, and very old established trees are enormous. As the get older they move away from the conifer-like sillouette of the younger plants, looking more like speading canopy trees, but such old plants are relatively rare.
Casurinas add to the already amazing inflammability of Australian bush, with the dried needles decomposing very slowly and building up under trees into a thick mat that ignites with the ease of petrol. Combine this with the thick, waxy, dry, volatile-oil filled leaves of the eucalyptus trees and you have a forest that is very, very easy to burn down.