Nardoo (Marsilea drummondii) is a native Australian fern that loves the water and grows wild in creeks and billabongs.
You can identify Nardoo by its characteristic 4 lobed leaflets that float on the water or above it on slender stems. The lobes close together at night.
The sporocarps (structures that produce and release spores) are edible. They’re bland and include a compound (thiaminase) that destroys vitamin B1, so they need to be prepared properly by baking.
Traditionally, they have been ground into yellow powder which was used as flour which was then baked, destroying the thiaminase.
The famous case of Australian explorers, Burke and Wills is an example of how this bushfood can kill if not prepared properly. Rather than carry on about Burke and Wills, I’ll refer you to this page where their case is dealt with on detail.
The leaves can be eaten raw but contain Oxalic acid so shouldn’t be eaten in large quantities.
Correctly prepared Nardoo will make you feel full, then but not give you very much nutrition at all. Poorly prepared flour can be downright hazardous to your health. I’d advise not eating it at all unless you’re stuck in the bush somewhere and in dire straits.