After doing the Bunnings run and being offered deck cleaner with a mix of oxalic acid and hydrochloric in liquid form then told no we don't have it in flakes @ 100% I did some research. Went back today and told them to learn their stock lines. This is what you are after at Bunnings. Go to the concrete cleaning area near the timber area.

I'd be very interested to see if this is like the "Lightning Rust Remover" that is available in the USA ? You leave the rusted legs to soak in in a long container and the next morning they look new.