The pilot was returning to Mackay after a business flight to Brampton Island. During the pre-flight checks at Mackay there had been an excessive drop in engine revolutions during the magneto checks, but this had cleared. A similar check conducted by the pilot when taxiing at Brampton Island indicated satisfactory magneto operation. During the takeoff run, just before lift-off, the right engine surged. The pilot rejected the takeoff. The rate of deceleration was reduced when the pilot used excessive braking, causing the mainwheels to lock. The aircraft overran the runway and was damaged when it crossed a concrete drain. The pilot reported that the fuel flow indication had surged during the take-off roll, but was unable to recall whether the surge was an increase or a decrease. Spark plugs and engine fuel system components were examined. The condition of the spark plugs removed from the no.6 cylinder indicated that the fuel mixture to that cylinder was excessively rich. However, the reason for the engine surge was not determined.