Should shelves match Cabinet Depth

I'm going to have two overhead cupboards in my Laundry and the space that I will be left with will be 233mm and I want to put two or three thick timber shelves in the space.

The cabinets are 300mm deep, should my shelving match the depth of the cabinets to make it all look flush and neat or does it really not matter?
I'm having trouble finding shelves that are 300mm deep that are nice and chunky.
Bunnings have some but are only 16mm and I'm not too keen on those, I'm more after 25mm similar.

I personally think it would look better but keen to see ideas on how to make it work if I can't get the depth to match.

Hi again Wendlle.
The best thing would be custom cabinets that fill the space, but I know you are using flatpack so the shelves are the next best option.
Normally the shelf would come to the same depth (actually a couple of mm behind) as the end panel but as it's so narrow it's probably not the end of the world.
What do you plan on using the shelf for?

yes thanks this is what I am thinking would look best.
The shelf is really just for decor items, I just don't have room for a cabinet there so want to fill the space rather than have it empty and looking unfinished.

Thru my work I can get melamine sheets, I'll see if I can get some in a faux timber finish instead of the normal white. I was really hoping not to go for the standard 16mm and wanted something thicker, will see how I go.

My next issue will be getting them to "float" there will be a wall on one side and a cabinet on the other. I really don't want to have the shelves between the two cabinets, and I didn't want to use brackets, but might have to.

I think I have a mental picture of what you are trying to do and I think you could achieve floating shelves with an unconventional approach and some effect.

If you could create custom brackets that attach to the wall but made from the same shelf material, these brackets should have pins or dowels that fit into the each shelf. The shelves may need oblong holes. These custom brackets would go attach the brackets to the wall and effectively become part of the shelf. Then on the third side screw from the inside of the cabinet to lock the shelves into place.