VH-LBA was maintaining FL 270 and northbound from Meekatharra. VH-FKI was southbound to Meekatharra and was cleared to descend from FL 310 to FL 260 through the level assigned to VH-LBA. The duty air traffic controller was advised of this error by the pilot of VH-LBA and VH-FKI was assigned a new clearance which should have provided lateral separation for the descent. However, the controller's calculations were in error and the distance between the aircraft was reduced below the minimum separation standard. The controller had just completed a hand-over/take-over when he was presented with this slightly more complicated separation problem. Coupled with this was a reduction in currency due to administrative tasks associated with being a team leader. Flight progress strip management practices at Perth may have contributed to the occurrence. Strip procedures have since been modified to be consistent with an apparent national standard developed from discussions with other check control staff.