If you want to smoke your food you can do so by adding various types of wood chips.  Hickory, mesquite, tea tree, cherry, and banksia wood chips can all be used.  You can also add herbs and spices to the mix for extra flavour.  NEVER use treated wood when barbecuing

Firstly you will need to soak the wood chips in cold water so that they will smoulder rather than burn.  

If you are using a gas barbecue put the soaked wood chips and/or herbs and spices into a smoke box over the burner while you are heating the plate.  Once the box starts to smoke you should turn down the burner so that the food will cook slowly which is best for smoking.  

If you are using a wood barbecue then you need to put the smoke box under the food.