Two words.... Fire Ban.
You can use Gas but cannot use Solid Fuel during some Fire Bans (may vary from state to state)
During a Total Fire Ban you cannot light or use a fire in the open air.
Solid fuel
You cannot use any BBQ or cooker that requires solid fuel such as wood or charcoal.
Gas
You can use a gas BBQ for cooking if it has an enclosed flame and all flammable material is cleared five metres away from around your BBQ.