Let me start by telling you that you have a beautiful garden! I brought in the expert to look at your question and offer an opinion, the hubby is a bricklayer and has plenty of experience with retaining walls. You, unfortunately, have way too much soil that needs to be held back to prevent the wash out that will occur without a full retaining wall. The terraces would erode rather quickly if you just had retaining blocks stacked loosely. Their strength is in having them stacked tightly with the gravel interlocking them. Sorry to disappoint. Good Luck, Tracy & Ed

I use those 4' orange electic fence stakes from home depot (about $1 a piece)for lots of things in the yard & gardens. Pound it in the ground and then slide the PVC pipe over it and into the soil a bit. Set out several of these with holes drilled in the PVC pipe and string wire through them to make a fairly sturdy, portable & cheap trellis. I use these stakes to build easy fencing. I use plastic cable ties to secure wire or plastic mesh fencing to them. Because I dislike the orange and it stands out like a sore thumb, I paint my stakes black. Pound 2 rows of these stakes in the ground about 4' apart and slide each end of a flexible PVC pipe over them, making an arch. Cover this with garden cloth or plastic to protect plants. I "sew" the fabric to the pvc with fishing line. If you are using plastic, cut some flaps in it, to keep it from blowing away in a big wind. I use them to brace non secured retaining walls. At my old house I built a raised bed using them to hold the lumber together. I drilled holes in the timbers and slid them over the stakes, then I lined the interior side with heavy plastic.

Re. frost line: Since what my mason is doing is basically a dry stack wall; from here: http://www.doityourself.com/forum/lawns-landscaping-outdoor-decor/451549-frost-line-retaining-wall.html#b A dry stack wall does not matter. The joints have enough space to allow this heaving movement. The large spacious joints in a dry stack wall are like expansion joints in other strucures, they will take movement.

Had to think about this for a while. I have never lived in a warm climate (much less Queensland), so I don't know your plants. You also said you like a formal garden look. To me, with just pictures and a trip or two to Florida to go by, "formal" and "tropical" are opposites. Formal is quiet; tropical is flamboyant. Formal is controlled, tropical is wild. Formal is geometrically ordered, tropical is plants with definite minds of their own, growing every way, everywhere. So how does one make a formal tropical garden? Here's an attempt. I matured the hedge, made a seat on part of the lower wall, as you proposed, with an extra raised planter for a back to support pillows. I "grew" your small climber (bougainvillea?), pruning it into a living artwork on the dark fence. The climber is bright red, so I eliminated the bright purple aubrieta, kept the Silver Falls dichondra. (I would at least try one small start of it at the base of the fence, giving it as much soil as you can, and see how it does. If it can't find enough resources to survive, you haven't lost much, and if it does well, you could then get more.) I added other touches of red, silver/green, and blue/gray, deliberately choosing some plants (sedum Sunsparkler Dazzleberry, near the patio and behind the bench) that repeat the fence color to integrate it into the garden. I noticed a very young tree in your photo, so I "grew" that, too, and planted around it (star jasmine, counting on some shading for it eventually from the tree), but I don't know what it is so didn't add leaves (and the growth pattern shown may be all wrong). Other plants left to right are Westringia fruticosa 'Grey Box', Alcanterea 'Silver Plum', a kangaroo paw, a grevillea beside the bench. Choices made mostly visually, with very little research; please forgive my ignorance of appropriate plants for Queensland and make your own choices! But maybe some visual ideas to consider. And just to satisfy my curiosity, what is the object on the left that looks like an enormous plant pot?