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Hello @Ella29
Welcome to the Bunnings Workshop community. It's marvellous to have you join us, and thank you for sharing your question about building a rock wall garden bed.
A basic key element when building a rock wall garden are the size of the stones. Multiple sizes will make it difficult to line up the stones as it will take a fair bit of time when doing a trial-and-error placement. It can be done in this manner, but you'll need to be patient when doing this type of assembly.
Another method I would like to suggest is to build a free-standing gabion wall. This is basically made of wire mesh in a rectangular shape. If you have smaller stones in your collection, you simply place a smaller wire mesh inside the gabion cage to prevent the rocks from coming out.
I suggest having a look at this discussion - Freestanding gabion walls by @Dave-1. There are some excellent examples of how to build a gabion wall. Since you only need it to be 1/2 a meter tall, you won't need to use a great deal of rocks.
Let me call on our experienced members @shevysingh, @Xfarma21 and @tony1982 for their recommendations.
If you need further assistance, please let us know.
Eric
Good morning @Ella29
I love gabions (Take a look at my bookmarks or search in here for gabions) or follow @EricL 's link 
Do you have a photo of the area you want to build the wall?
I have made multiple types of walls, some retaining, some frestanding and some for windbreaks (A few to mark a boundray line) Bunnings sells premade cages Gabion cage They sell three different sizes and also two different colours. The pre-made ones go together remarkably easy. I step through the process in one of my projects.
With the cages I actually "face" the mesh with sandstone rocks, with the larger blocks of sandstone I have hit them with a hammer/pound hammer (small sledge hammer) and broken them up. The colour looks amazing and will stay for years. (I have had one wall for a decade and its only now I am thinking of pressure washing it even tho it donst really need it to bring back the brightness) The non seen part of the cage I fill with all my waste style rocks.
I have lined the space between the soil edge and the cage with some old upholstery material (You could use any course cloth) to stop the soil from coming through into the cage. The best thing about the cages is that you get to play "tetris" whith the facing stones. I also like the cages as I can shift them/change them if I want. There are no concreteing issues and they dont need foundations (depending on size)
Currently I have a mango, olive tree? and some rosmary growing behind one of my retaining walls. They seem to be doing well. I alos have been growing tomatoes in front of thenm and have only just harvested the last of them last week. (Theory is the heat from the conncrete in the cages helps keep frost away and also extends the growing season)
If you dont like gabions (Oh No! lol) my suggestion would be to make sure you have drainage in the base of your wall. Make sure there is somewhere for the water to drain to and break up the larger sandstone stones. The fresh colours will be amazing!
Dave
