The wombat test is a computer based game you play where you are given a variety of tasks to do, and you will score points based on how well you can manage and prioritise tasks given to you. There is one main task which you must always monitor, then there are whats called bonus tasks, which you undertake to score more pointsl. Its quite fun actually!
If you are required to do the wombat test for stage 1, then they will send you an instructions booklet (not everyone will do it in stage 1, some will do it during stage 2, just depends on how they have structured your interview day, if in doubt always email them to confirm
)