To have the GREEN LIGHT is to be given the ALL CLEAR by the Australian police to conduct your criminal business, usually in exchange for money or information. The 'green light' is often given to high profile hitmen who specialise in killing crime bosses or important drug dealer's who pay money or give information in exchange for immunity.
One such case was seen in the 80's when the 'green light' was given by corrupt cop Roger Rogerson to notrious criminal and drug dealer Neddy Smith. A movie 'Blue Murder' was subsequently based on this.
The 'green light' is a very powerful but dangerous tool, for both the police and any criminal who recieves it. The probem for criminals such as Neddy is that the light can be turned 'red' at any moment.