QANTAS A330-200 VH-EBO operating QF581 Sydney – Perth encountered a loss of separation with sister VH-EBS operating QF576 Perth – Sydney this afternoon. VH-EBO, cruising at 38,000 ft, was cleared to climb to 40,000 ft. This caused vertical separation with VH-EBS, which was cruising at 39,000 ft, to reduce to 700 ft. After a  TCAS alert VH-EBO descended and VH-EBS ascended to re-establish separation.