When someone is told to "bring a plate" to a picnic in Australia, does it literally mean just bring a plate? Or should they be bringing something else as well?
No. You do not want to turn up with just a plate - ie a piece of crockery.
Bring a plate is to bring a plate of food that can be eaten from said plate. This can be either a sweet food such as pikelets or chocolate crackles; or savoury, such as triangle sandwiches, potato salad etc.
Less common is to bring a large dish, such as quiche or salad that needs a bowl rather than a plate, and even less common is to bring something that would serve as an entire course, ie roast lamb with vegetables. Not entirely unknown, just less common.
As a meme, bring a plate arose as western social gatherings in Australia traditionally revolve around cartons of beer and meat being cooked on a barbecue with the occasional salad. Bringing a plate added some involvement for the womenfolk, and the food kept the kids from going hungry - allowing the men longer time to keep drinking. :-)
This is an example of synecdoche, a figure of speech which uses part of a thing to mean the thing itself. In this case the word plate (or, in other locales, the dish) means a plate of food.
Synecdoche is used frequently in English. This is equally true of other languages. That can create a challenge when learning a new language, because it is not easy to imagine all the ways a word might stand for a part of something else.