I'm not sure what State you are in or what lines were marked on the road.
I've just been through this in Victoria; I believed I was in the right. I merged left to right in stop start traffic; the car behind me wanted to block the car behind them and got too close to me. The hinge on the back corner of my trailer opened up their front guard like a can opener.
My insurance company deemed me at fault quoting the following road law;
http://www5.austlii.edu.au/au/legis/...7208/s148.html
148 Giving way when moving from one marked lane or line of traffic to another marked lane or line of traffic
(1) A driver who is moving from one marked lane (whether or not the lane is ending) to another marked lane must give way to any vehicle travelling in the same direction as the driver in the marked lane to which the driver is moving.
In my instance there were lines on the road, so it was deemed my fault.
If no lines are marked on the road then road rule 149 covers it and the car in front when merging has right of way.
My insurance company decided to put the claim on my trailer policy so the excess was only $50, not worth me arguing.