In NSW, what is the 'zip merge' (or 'zipper merge') rule when a lane ends and traffic must merge?

1. The vehicle behind always gives way to the vehicle ahead
2. Drivers in both lanes alternate to take turns merging — one vehicle from each lane at a time
3. The driver in the ending lane must always give way to all vehicles in the continuing lane
4. No rule applies — drivers must negotiate as they see fit