Rainforests are different from other Australian forests in that they have dense canopies of leaves that prevent most sunlight from reaching the ground. Ferns, vines, epiphytes, lichens and mosses abound. Rainforests are closed forests in contrast with open forests dominated by eucalypts and wattles. Mixed forests are where eucalypts tower over rainforest plants.

Rainforests grow at Robertson because some ideal conditions exist, including: soils that are structurally ideal for tree growth and derived from basalt, higher and more reliable rainfall than in most parts of NSW, frequent fog and fewer bushfires. Basalt is a volcanic rock that contains more nutrients than the surrounding shales and sandstones. 

In New South Wales there are different types of rainforests, each with distinctive species and characteristics. Robertson Rainforest is the warm-temperate type but include some cool-temperate species. Our temperate rainforests have Gondwanan origins. Similar plants grow in Victoria, Tasmania, New Zealand, Lord Howe Island and Chile, as well as on the mountains of Queensland, New Caledonia, and New Guinea. Fossils of similar plants occur in Antarctica.

Most of the coastal rainforests in the Illawarra are subtropical; a remnant is at Minnamurra Rainforest. A type of subtropical rainforest called littoral rainforest grows close to the sea, one example being at Bass Point. In areas of lower rainfall on the Razorback Range and in the Shoalhaven River gorge there are dry rainforests and vine-thickets. 

Cool-temperate rainforests have only one or two dominant trees; local examples are Pinkwood and Possumwood. Our warm-temperate rainforests have more tree species, the most common being Sassafras, Lilly Pilly and Coachwood, far fewer than subtropical rainforests. Other species are unevenly dispersed.

The canopy is fairly even and lower in rocky and windswept places. A layer of smaller trees and shrubs is below the canopy. Leaves are mostly simple, medium-sized and often toothed. Trunks do not have buttresses and are often single, but some older trees have coppiced, forming a ring of trunks around the original dead or cut trunk. Such trees may be centuries old. Large woody vines are a distinctive feature at Robertson, unusual in temperate rainforests.

Ferns and other herbaceous plants sometimes grow in profusion on the forest floor. Epiphytes, such as orchids, ferns, mosses and lichens, are common. Palms are rare. Strangler-figs, a feature of warmer forests, do not occur. Remarkable local features are rainforests on exposed northern and western slopes, unusual in Southern NSW, and the growth of rainforests dominated by Coachwood on rich soils. Rainforests here also grow on the less-fertile grey shale soils adjoining the red basalt because nutrients have washed down to them.

The Treeferns have not been included in this site. The two common ones around Robertson are the Rough Treefern, Cyathea australis, and the Soft Treefern Dicksonia antarctica. The thin-trunked and very aptly named Prickly Treefern, Cyathea leichhardtiana, does occur in some places but is not at all common.