During the landing roll the aircraft steering could not be controlled by the steering tiller. The tiller continued to drive to the full left position and directional control could only be maintained by differential braking. When the aircraft was stopped the nosewheel and tiller were locked in the full left position and taxiing was not possible. While arrangements were being made to disembark the passengers on the runway, the steering suddenly freed itself and the aircraft was then taxied to the terminal. The steering system could not be faulted on the ground at Albury but the nosewheel steering shut-off valve was changed as a precaution. The aircraft then operated the return flight to Sydney, but the problem recurred. The nosewheel steering control valve was changed and no further problems have been reported.