Nardoo (Marsilea drummondii) is a native Australian fern that loves the water and grows wild in creeks and billabongs.
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 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.