That looks great @Jason i will head to the big green shed tommorow and see wht is available. thanks everyone for the ideas.
I'd consider treated pine or railway sleepers.
You'll find lots of examples on Workshop, like @ProjectPete's garden beds here - https://www.workshop.com.au/t5/Gardening/rustic-railway-sleeper-garden-bed/m-p/432/highlight/true#M4...
@maknilsin garden edging here - https://www.workshop.com.au/t5/Gardening/Backyard-Makeover/m-p/15109#M1926
@kel has some edging here - https://www.workshop.com.au/t5/Gallery/the-vertical-gardens-that-i-built/cns-p/13903
There's also been a bit of discussion about composite sleepers, like here - https://www.workshop.com.au/t5/Gardening/Building-a-raised-vegie-garden/m-p/5248#M718
Looking forward to seeing how it comes together @Gezza87.
Glad to see the community was able to provide assistance with the project.
By the way, I also ordered those river pebbles in my photo from the Bunnings special orders desk. You can by them by the square metre and have them delivered if you need a lot (like we did).