Thanks @Adam_W Yes it does. I might need to check first where digging the posts as it runs close to a sewer line I think. How much concrete would I need for each post? I figure you mean about .2m x .2m x .5m x 4 posts? Ie about 20L per hole?
Thank you for the update on your proposed pond. Let me tag @Adam_W so they're made aware of your question. Looking at Adam_W's recommendation, I read it as 500mm wide in diameter and 500mm deep for each post. It's recommended to be built that way to hold both the retaining wall and the water in your raised pond. Using a standard 20kg bag as our reference, you would approximately need 11 bags per post hole.
I agree with Adam_W's recommendation of high MPA-rated concrete. This type of concrete is at least twice as hard as regular concrete. If you would like to tell me your closest store, I can inquire for you to find out what they have in stock and have some put aside.
If you need more advice or information, please let us know.