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Hello @damianpc
Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about your concrete benchtop.
In regards to using timber as the bottom support for the legs, I suggest engaging the services of a registered builder. They will be able to design and build you a proper support structure for the concrete benchtop. In regards to cladding the timber support in James Hardie 3000 x 325 x 14mm Stria Standard Profile Cladding, I'm not aware of any reason why it won't work. Simply follow the James Hardie Stria installation guide to make sure that it fits the timber frame properly.
It would be better to leave the bottom unsealed allowing air to pass through the structure of the table. It would give the framework a chance to breath and dry off. As to the type of timber to use, I suggest using 90x45 or bigger and H3 treated for outdoor use.
Let me call on our experienced members @Dave-1, @Nailbag, @Jewelleryrescue and @JoeAzza for their recommendations.
If you need further assistance, please let us know.
Eric
Good Morning @damianpc
That will be a nice looking outdoor island bench 
I like both @EricL's and @Nailbag's suggestions but like Nailbags steel support even better. Mainly for long term weight stability. Even 3/4cm depth of a concrete top is a fair amount of weight to be a base load. Think of how many concrete bags you will need to cover the bench at 3-4cm and you may be surpprised at the weight (even without the water added)
What are you considering having underneath the concrete and left in place long term? Concrete formwork?
One last thought, is the bench going to be sitting on a concrete area or a deck?
Dave
Morning @damianpc
I havnt worked with steel before as in welding either However I have worked with steel as in bolting ect You could treat angle iron the same way as welding but via bolting. It may not have the ridgidity tho, that would depend on how you would construct it.
Sketching up the design of your bench might be an idea, depending on the materials you use it could really help decide the path you want.
With the "small concrete pad" you are installing for the bench. I would also suggest to sketch out the whole area as the concrete footing wont be going places easily you could put footings in to support the bench instead of a pad if you want a deck down the track. If you level off the footings that way you could easily reuse them if you decide to shift the bench.
Dave
Hi @damianpc,
A concrete bench with a 2m length is a large span, and the weight of the concrete top needs to be properly supported to avoid cracking or failure over time. Bricks or concrete blocks could certainly be used for the supports, but the key factor is how the weight is distributed.
If your supports are positioned at the ends only, then a 2m span would require significant reinforcement, such as metal lintels running through the concrete. Standard reinforcement rod (reo rod) alone is unlikely to provide enough structural support to prevent cracking or eventual collapse.
It would be beneficial to post a sketch of how you plan to support the bench, including details on how the concrete top will be reinforced. Given the weight and potential safety risks if the bench is incorrectly built, I would strongly advise enlisting the help of a professional—even if just for guidance on structural integrity. A structural engineer or builder could provide valuable advice to ensure your design is safe and durable.
Mitchell
Thanks for that. I've tried to sketch out what I was thinking. It's basically a 3 dimensional mini stud wall. Using 75 x 200 treated pine. Cross bracing both front to back and horizontally left to right. It's a little hard to sketch it the way it is in my head.
Hello @damianpc
That is a good sketch of the timber support. I suggest bringing your sketch to an engineer or builder and explaining to them what you plan to build and put on top of the timber frame. Since you have the length, width and height of the benchtop you're planning to build. The engineer and builder should be able to give you advice in regards to the timber base and the proper construction method for the concrete bench top.
If you need further assistance. please let us know.
Eric
