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 macadamias, garlic, rosemary, lemon myrtle, salt, pepper and olive oil into a chunky paste.
Spread the Dijon mustard over flesh of the lamb racks, press the macadamia crumb onto the outside of the racks of lamb. If desired, wrap the bones in alfoil.
Once the barbecue has preheated, make adjustments to the burner control(s) for the roasting setting. Place the lamb racks on and cook for 20 to 30 minutes or until cooked to your liking.
Leave to rest for 5 to 10 minutes before carving into individual portions.