Set up your barbecue for indirect cooking. Please refer to your handbook for further instructions. Preheat your barbecue for roasting.
Using a sharp knife trim most of the fat from the lamb rack, leaving behind a thin layer.
In a food processor, process the nuts, garlic, rosemary, lemon myrtle, salt, pepper, and olive oil into a chunky paste.
Spread the Dijon mustard over the flesh of the lamb racks, and press the macadamia paste onto the outside of the lamb racks. If desired, wrap the bones in alfoil.
Once the barbecue is preheated, make adjustments to the burner control(s) for the roasting setting. Place the lamb racks on and cook to your liking, 20 to 30 minutes. Remove from the barbecue and leave to rest for 5 to 10 minutes before carving into individual portions.