Good question Kerry. Yes and no. Depends on the plant in question.
Good example is mushrooms - there's a yellow mushroom that tends to grow in pine forests, that is apparently really nice. There's another mushie that looks practically identical, that will make you sick.
Most of the plants I've put in this thread though, are pretty easy to spot once you have your eye in, and don't seem to have any significant doppelgängers - or at least ones that I'm aware of, that are likely to cause a problem... eg: The native cherry is vaguely similar to a juvenile bribie island pine in terms of foliage. However, the pine doesn't have the fruit, so it's pretty hard to mix them up.
There are a few plants I've specifically avoided adding to this thread, because they're easy to confuse, they're too damaging to collect, there are potential health problems, or they're dangerous if not prepared correctly.
Couple of examples:
* Tree fern, rock orchids, and most palms (you can eat the heart out of these, but it generally kills the plant)
* Native ginger is a bit hard to pick sometimes.
* There's a bit of uncertainty as to how safe one of our native hibiscuses is to eat - there has been one report of kidney problems after drinking tea made from the leaves of one of the plants ( http://www.hibiscus.org/species/hheterophyllus.php
* A couple of our native nuts have really nasty hairs inside the shell, which require particular preparation to get rid of.