Allocasuarina portuensis, commonly known as Nielsen Park she-oak,[2] is a species of flowering plant in the family Casuarinaceae and is endemic to a restricted part of the Sydney region in eastern New South Wales.

It is likely that the species previously had a wider distribution, since the vegetation of Nielsen Park at the time of European settlement was widespread on the foreshore of Sydney Harbour, but has since been extensively cleared.

At the time of its discovery, the species was growing in tall, closed woodland with Port Jackson fig (Ficus rubiginosa), smooth-barked apple (Angophora costata), blueberry ash (Elaeocarpus reticulatus) and cheese tree (Glochidion ferdinandi) in the canopy, and an understorey including Pittosporum revolutum, Kunzea ambigua and Monotoca elliptica.

The original habitat was degraded by weeds, such as asparagus "fern" (Protasparagus densiflorus), wandering Jew (Tradescantia albiflora) and Lantana camara.

The original vegetation may have been more sclerophyllous, the absence of recent wildfire allowing the presence of more mesophyllic species, such as Glochidion ferdinandi and Pittosporum revolutum.