BBQ gas cylinders will not be carried by a removalist unless you can provide a certificate from a Gas Cylinder Test Station certified by Standards Australia, stating the cylinder has been purged. You should never burn off the gas nor simply release the gas into the atmosphere. Gas is heavy and will not dissipate making an explosion a distinct possibility. The BBQ should be cleaned before moving day.
Lawnmowers and other gardening equipment should have the petrol drained from the tanks. If you are sending these items interstate or into store, they will also need to be steam cleaned.