The calendar year aspect can be dealt with given that the OP's travel patterns would indicate that he may well have 2 trips to perth within a 3 month period. 2 trips to Perth can complete the platinum challenge

Enrol in the challenge with a start date on or after 16 june, then status will count for rest of 2013 plus 2014 , so no need to worry about getting 50k points/miles in the 6.5 months remaining of 2013

I would recommend switching now and getting a challenge underway after mid june

Of course, doing the challenge will also reduce the number of trips where there are no bonuses/minimum mile guarantees