I guess the 45 weeks gives some time for a veteran to go through some rehabilitation while they continue to receive the 100%. It probably has no real science behind it, but is something that we inherited from a workers' compensation scheme for Australian public servants, and it is steeped in history. It is currently being reviewed and may change. The government may be interested in some other recommendations before it, to perhaps have a more graduated step down so that it's not such a sudden step from 100% to 75%.
On June 10th, 2013. See this statement in context.