We bought 3 raised garden bed to be located on concrete. I would like to know what kind of lining we use inside. We don't need a weed-net as there are no weeds. Thanks.
Welcome to the Bunnings Workshop community @patricial. Many thanks for your question, I'd be pleased to assist.
You might be interested in reading the article by the fabulous @Noelle on How to fill a raised garden bed. In step two she illustrates the use of weed mat and geo fabric which will allow water through but not soil or potting mix particles. The geo fabric ensures the water leaking out on to your concrete is clean and will minimise any discolouration to the surface.
Through its versatile design weed matting has many uses other than just controlling weeds. Its fine mesh allows water to penetrate yet stops weeds, roots, pests and soil moving through it.
Using geo fabric or drainage matting is a common way to secure all the potting mix within the raised garden bed. You'll notice @Adam_W also utilising this technique in step six of How to build a raised garden bed.
Although these articles are both referencing timber garden beds the same principle can be applied to your Birdies raised garden bed.
Please let me know if you need further assistance or had other questions.