Thank you all for your comments. Well, it's been a few months since my original post and we have since finished putting up the retaining wall and planting the hedge. We decided to go ahead with Murrayas - planted about 34-40 cm apart. I gave them a good trim about a week after planting at the end of Autumn and another quick trim at the start of Spring. They have really filled out in warm weather and I think the gaps between individual plants should close by the start of winter. I'm really happy with the choice - the hedge looks glossy and lush and the sweet smell of Murraya flowers is just beautiful. We didn't do any underplanting, but I'm still thinking about putting in the dwarf agapanthus for the inside border. A couple of weeks ago we planted some Rhaphiolepis "Snow Maiden" and Rhoeo along one side of the retaining wall to provide a border for a footpath and I'm really happy how it turned out.

Thanks all for your ideas and suggestions. We do plan on waterproofing :) I’m in Perth and the area gets probably about 6-7 hours of full morning sun and then part sun in the afternoon. The Murraya peniculata looks nice, does it have a nice smell? Could I put a smaller plant or hedge in front of it? And how often does it need to be watered? Another idea was to put star jasmine on a trellis and a smaller box hedge in front. Or those plants that look like a hedge ball (if that makes sense haha). Thanks again :)

Wow amazing advice - thank you!! We are in the mid-hinterland on the Gold Coast. :) Since posting we have planted out the back strip with a mix of Vibernum, Chinese dwarf bamboo, elephant ears, Xanadu and ornamental grass. We still have both sides to do yet and it feels like it will never end! Unfortunately our garden beds were edged with pavers at build stage and we aren't equipped to rip them out to widen the garden beds now. but I am considering culling the mulch in some areas on the sides and putting in feature sections with Riverstones. I love the idea of using water feature/bowl and will try to incorporate one. We went and bought 9 golden cane palms which we haven't yet planted due to some posts online about the root systems and them being problematic, we are thinking about maybe putting them along the retaining wall, any ideas how to manage the root growth? We thought about keeping them in larger pots and planting around the pots, has anyone seen this executed well? Updated pic attached. You can see a bush fire that went across the hinterland yesterday, bit close for comfort! Also the palms we are thinking about in bags against the wall. PS - Whoever would have thought that turf is so hard to keep lush! OMG thats a whole other post! hahaha

Pop over to Pintrest and search “Pleached trees with hedge underneath”, I suspect this is the look your landscaper is looking for. Eg below. Plenty of examples of trees planted amongst hedges. Rather than ornamental pears, have a look at grafted/weeping/standard grevilleas. They fit with your native hedge and will have similar water and food requirements plus they give you some height straight away. And have foliage all year around.