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I did a bit of research to find suitable materials for this project, so here's a quick overview that others can hopefully find useful.
Here's how the tabletop looks after coming out of the form and a quick sand.
To make the form, I used this melamine https://www.bunnings.com.au/2400-x-1200mm-16mm-melamine-particleboard-white-matt_p0097412
The concrete I used is this one: https://www.bunnings.com.au/easy-mix-20kg-55mpa-super-strength-concrete_p0760460
This reomesh https://www.bunnings.com.au/jack-1800-x-1050-x-5mm-light-duty-reo-mesh_p1060321
Silicone spray to ensure no sticking to the form https://www.bunnings.com.au/wd-40-300g-specialist-lawn-garden-water-protective-silicone_p3410102
Silicone for rounding the corners of the form https://www.bunnings.com.au/parfix-roof-and-gutter-silicone-300g-black_p1232632
Paint scraper set for rounding the silicone in the form https://www.bunnings.com.au/paint-partner-4-piece-silicone-scraper-set_p1660196
Cut the pieces of melamine, I made it so the concrete will be 40mm thick. The size was 2m x 1m. I needed 8 bags of concrete for this size.
Then screw the sides on.
Then run a thin bead of the silicone around the edges and round them using the scraper set.
This will mean the corners of the tabletop will be nicely rounded.
Prior to adding the concrete, ensure the melamine is clean, then spray all over using the silicone spray.
Mix up the concrete according to the instructions on the bags, I had to add an extra couple hundred mils of water to what it said to get it a nice consistency.
Fill the form up half to 3/4 with concrete.
I left the edges a bit longer so it was easy to grab onto to pull them off later.
Then you'll want to vibrate it a lot on the sides and bottom. I used a reciprocating saw with no blade in it, and ran it around the sides and underneath for about half an hour. You will see all the bubbles coming up. But otherwise you can use a rubber mallet, or even a hammer, you may have to do it longer to get good results.
Then add the reomesh. I needed slightly more than just the one sheet for the length, so I cut a bit off another sheet I already had.
Then fill up with the remainder of the concrete.
After that, vibrate again for as long as there's still bubbles coming up. I smoothed the surface a few times and vibrated again. Probably 45 mins or so of vibrating.
Then that's it for the casting, just smooth the surface as much as you want (it will be the bottom once you flip it, so it doesn't matter too much).
Cover it with plastic so it dries slowly. Might need a light spray each day for a couple of days. Then plastic back on.
Then remove the sides and flip it and remove the top. Sides and top came off super easy.
Then I wet it down and used 240 grit wet and dry to sand it, then 400 grit for final wet sanding.
After that I sealed it using penetrating sealer.
I'm super happy with how it turned out. It's really strong, doesn't have any flex when you lift it from the two ends.
Hello @markw
Thank you so much for sharing the photos and steps you took to build this amazing concrete benchtop. Often these types of benchtops have a melamine panel support underneath, which makes yours truly special as it can stand on its own. Can you please tell us the brand of sealer you used to achieve such a polished finish?
Again, thank you so much for sharing such an inspirational project.
Eric
Good afternoon @markw
Far out! How good did that tabletop turn out! 
Ive wanted something like this for a long time but havnt seen a something as smoothly done as yours. Really worth having a go I'd say. My timber bench idea for my kitchen may have just been bumped!
I really liked your explanation of how to do the project.
Thanks
Dave
Hello @raygun
Welcome to the Bunnings Workshop community. It's wonderful to have you join us, and thank you for sharing your question about the concrete tabletop.
Let me tag @markw to make them aware of your kind words and question. I believe that the type of concrete used, and the Rio imbedded in the concrete has made it extremely strong. Plus, if you had several extra hands holding the table while being turned over would protect it from cracking.
Please make sure to post an update on your project. I'm sure our members would be keen to see how you'll be building your concrete benchtop.
If you have any other questions we can help with, please let us know.
Eric
Hello @garytwood
Welcome to the Bunnings Workshop community. It's sensational to have you join us.
One of the things you need to look at is the MPA rating on the bag, if it says 40MPA+ and above it is usually high strength. The only other ones I can find are the MaxPRO 20kg Concrete Mix and the Easy Mix 20kg Supremo-32 High-Strength Concrete Mix 32MPa. I also suggest speaking to a timber yard team member at the store as they will know what's available for your project.
If you need further assistance, please let me know.
Eric
