Michael, you can go down right upto 2lb test provided you have a reel with a silky smooth drag. It also depends on whether you have snags in your area. In this case you would have to use a minimum of 8lb test.
Bops is right... depends on the snag quotient, I've landed a 2.5 kg guy on 2 lbs line in clear water, but then again, there are places with big guys I would not touch with a 8 lbs line!!! but a 3 kg guy will be best fougth on max 4 lb line... murrel just has that initial fight... few fight long and hard... bigger challenge is keepng the hook in the guys mouth than actually landing him!